peewee.py

from bisect import bisect_left
from bisect import bisect_right
from contextlib import contextmanager
from copy import deepcopy
from functools import wraps
from inspect import isclass
import calendar
import collections
import datetime
import decimal
import hashlib
import itertools
import logging
import operator
import re
import socket
import struct
import sys
import threading
import time
import uuid
import warnings

try:
    from pysqlite3 import dbapi2 as pysq3
except ImportError:
    try:
        from pysqlite2 import dbapi2 as pysq3
    except ImportError:
        pysq3 = None
try:
    import sqlite3
except ImportError:
    sqlite3 = pysq3
else:
    if pysq3 and pysq3.sqlite_version_info >= sqlite3.sqlite_version_info:
        sqlite3 = pysq3
try:
    from psycopg2cffi import compat
    compat.register()
except ImportError:
    pass
try:
    import psycopg2
    from psycopg2 import extensions as pg_extensions
except ImportError:
    psycopg2 = None
try:
    import MySQLdb as mysql  # prefer the C module.
except ImportError:
    try:
        import pymysql as mysql
    except ImportError:
        mysql = None


__version__ = '3.1.7'
__all__ = [
    'AsIs',
    'AutoField',
    'BareField',
    'BigAutoField',
    'BigBitField',
    'BigIntegerField',
    'BitField',
    'BlobField',
    'BooleanField',
    'Case',
    'Cast',
    'CharField',
    'Check',
    'Column',
    'CompositeKey',
    'Context',
    'Database',
    'DatabaseError',
    'DataError',
    'DateField',
    'DateTimeField',
    'DecimalField',
    'DeferredForeignKey',
    'DeferredThroughModel',
    'DJANGO_MAP',
    'DoesNotExist',
    'DoubleField',
    'DQ',
    'Field',
    'FixedCharField',
    'FloatField',
    'fn',
    'ForeignKeyField',
    'ImproperlyConfigured',
    'Index',
    'IntegerField',
    'IntegrityError',
    'InterfaceError',
    'InternalError',
    'IPField',
    'JOIN',
    'ManyToManyField',
    'Model',
    'ModelIndex',
    'MySQLDatabase',
    'NotSupportedError',
    'OP',
    'OperationalError',
    'PostgresqlDatabase',
    'PrimaryKeyField',  # XXX: Deprecated, change to AutoField.
    'prefetch',
    'ProgrammingError',
    'Proxy',
    'SchemaManager',
    'SmallIntegerField',
    'Select',
    'SQL',
    'SqliteDatabase',
    'Table',
    'TextField',
    'TimeField',
    'TimestampField',
    'Tuple',
    'UUIDField',
    'Value',
    'Window',
]

try:  # Python 2.7+
    from logging import NullHandler
except ImportError:
    class NullHandler(logging.Handler):
        def emit(self, record):
            pass

logger = logging.getLogger('peewee')
logger.addHandler(NullHandler())

# Import any speedups or provide alternate implementations.
try:
    from playhouse._speedups import quote
except ImportError:
    def quote(path, quote_char):
        quotes = (quote_char, quote_char)
        if len(path) == 1:
            return path[0].join(quotes)
        return '.'.join([part.join(quotes) for part in path])


if sys.version_info[0] == 2:
    text_type = unicode
    bytes_type = str
    buffer_type = buffer
    izip_longest = itertools.izip_longest
    exec('def reraise(tp, value, tb=None): raise tp, value, tb')
    def print_(s):
        sys.stdout.write(s)
        sys.stdout.write('\n')
else:
    import builtins
    from collections import Callable
    from functools import reduce
    callable = lambda c: isinstance(c, Callable)
    text_type = str
    bytes_type = bytes
    buffer_type = memoryview
    basestring = str
    long = int
    print_ = getattr(builtins, 'print')
    izip_longest = itertools.zip_longest
    def reraise(tp, value, tb=None):
        if value.__traceback__ is not tb:
            raise value.with_traceback(tb)
        raise value


if sqlite3:
    sqlite3.register_adapter(decimal.Decimal, str)
    sqlite3.register_adapter(datetime.date, str)
    sqlite3.register_adapter(datetime.time, str)


__date_parts__ = set(('year', 'month', 'day', 'hour', 'minute', 'second'))

# Sqlite does not support the `date_part` SQL function, so we will define an
# implementation in python.
__sqlite_datetime_formats__ = (
    '%Y-%m-%d %H:%M:%S',
    '%Y-%m-%d %H:%M:%S.%f',
    '%Y-%m-%d',
    '%H:%M:%S',
    '%H:%M:%S.%f',
    '%H:%M')

__sqlite_date_trunc__ = {
    'year': '%Y',
    'month': '%Y-%m',
    'day': '%Y-%m-%d',
    'hour': '%Y-%m-%d %H',
    'minute': '%Y-%m-%d %H:%M',
    'second': '%Y-%m-%d %H:%M:%S'}

__mysql_date_trunc__ = __sqlite_date_trunc__.copy()
__mysql_date_trunc__['minute'] = '%Y-%m-%d %H:%i'
__mysql_date_trunc__['second'] = '%Y-%m-%d %H:%i:%S'

def _sqlite_date_part(lookup_type, datetime_string):
    assert lookup_type in __date_parts__
    if not datetime_string:
        return
    dt = format_date_time(datetime_string, __sqlite_datetime_formats__)
    return getattr(dt, lookup_type)

def _sqlite_date_trunc(lookup_type, datetime_string):
    assert lookup_type in __sqlite_date_trunc__
    if not datetime_string:
        return
    dt = format_date_time(datetime_string, __sqlite_datetime_formats__)
    return dt.strftime(__sqlite_date_trunc__[lookup_type])


def __deprecated__(s):
    warnings.warn(s, DeprecationWarning)


class attrdict(dict):
    def __getattr__(self, attr):
        try:
            return self[attr]
        except KeyError:
            raise AttributeError(attr)
    def __setattr__(self, attr, value): self[attr] = value
    def __iadd__(self, rhs): self.update(rhs); return self
    def __add__(self, rhs): d = attrdict(self); d.update(rhs); return d

SENTINEL = object()

#: Operations for use in SQL expressions.
OP = attrdict(
    AND='AND',
    OR='OR',
    ADD='+',
    SUB='-',
    MUL='*',
    DIV='/',
    BIN_AND='&',
    BIN_OR='|',
    XOR='#',
    MOD='%',
    EQ='=',
    LT='<',
    LTE='<=',
    GT='>',
    GTE='>=',
    NE='!=',
    IN='IN',
    NOT_IN='NOT IN',
    IS='IS',
    IS_NOT='IS NOT',
    LIKE='LIKE',
    ILIKE='ILIKE',
    BETWEEN='BETWEEN',
    REGEXP='REGEXP',
    CONCAT='||',
    BITWISE_NEGATION='~')

# To support "django-style" double-underscore filters, create a mapping between
# operation name and operation code, e.g. "__eq" == OP.EQ.
DJANGO_MAP = attrdict({
    'eq': OP.EQ,
    'lt': OP.LT,
    'lte': OP.LTE,
    'gt': OP.GT,
    'gte': OP.GTE,
    'ne': OP.NE,
    'in': OP.IN,
    'is': OP.IS,
    'like': OP.LIKE,
    'ilike': OP.ILIKE,
    'regexp': OP.REGEXP})

#: Mapping of field type to the data-type supported by the database. Databases
#: may override or add to this list.
FIELD = attrdict(
    AUTO='INTEGER',
    BIGAUTO='BIGINT',
    BIGINT='BIGINT',
    BLOB='BLOB',
    BOOL='SMALLINT',
    CHAR='CHAR',
    DATE='DATE',
    DATETIME='DATETIME',
    DECIMAL='DECIMAL',
    DEFAULT='',
    DOUBLE='REAL',
    FLOAT='REAL',
    INT='INTEGER',
    SMALLINT='SMALLINT',
    TEXT='TEXT',
    TIME='TIME',
    UUID='TEXT',
    VARCHAR='VARCHAR')

#: Join helpers (for convenience) -- all join types are supported, this object
#: is just to help avoid introducing errors by using strings everywhere.
JOIN = attrdict(
    INNER='INNER',
    LEFT_OUTER='LEFT OUTER',
    RIGHT_OUTER='RIGHT OUTER',
    FULL='FULL',
    FULL_OUTER='FULL OUTER',
    CROSS='CROSS')

# Row representations.
ROW = attrdict(
    TUPLE=1,
    DICT=2,
    NAMED_TUPLE=3,
    CONSTRUCTOR=4,
    MODEL=5)

SCOPE_NORMAL = 1
SCOPE_SOURCE = 2
SCOPE_VALUES = 4
SCOPE_CTE = 8
SCOPE_COLUMN = 16


# Helper functions that are used in various parts of the codebase.
MODEL_BASE = '_metaclass_helper_'

def with_metaclass(meta, base=object):
    return meta(MODEL_BASE, (base,), {})

def merge_dict(source, overrides):
    merged = source.copy()
    if overrides:
        merged.update(overrides)
    return merged

if sys.version_info[:2] == (2, 6):
    import types
    def is_model(obj):
        if isinstance(obj, (type, types.ClassType)):
            return issubclass(obj, Model)
        return False
else:
    def is_model(obj):
        if isclass(obj):
            return issubclass(obj, Model)
        return False

def ensure_tuple(value):
    if value is not None:
        return value if isinstance(value, (list, tuple)) else (value,)

def ensure_entity(value):
    if value is not None:
        return value if isinstance(value, Node) else Entity(value)

def chunked(it, n):
    marker = object()
    for group in (list(g) for g in izip_longest(*[iter(it)] * n,
                                                fillvalue=marker)):
        if group[-1] is marker:
            del group[group.index(marker):]
        yield group


class _callable_context_manager(object):
    def __call__(self, fn):
        @wraps(fn)
        def inner(*args, **kwargs):
            with self:
                return fn(*args, **kwargs)
        return inner


class Proxy(object):
    """
    Create a proxy or placeholder for another object.
    """
    __slots__ = ('obj', '_callbacks')

    def __init__(self):
        self._callbacks = []
        self.initialize(None)

    def initialize(self, obj):
        self.obj = obj
        for callback in self._callbacks:
            callback(obj)

    def attach_callback(self, callback):
        self._callbacks.append(callback)
        return callback

    def __getattr__(self, attr):
        if self.obj is None:
            raise AttributeError('Cannot use uninitialized Proxy.')
        return getattr(self.obj, attr)

    def __setattr__(self, attr, value):
        if attr not in self.__slots__:
            raise AttributeError('Cannot set attribute on proxy.')
        return super(Proxy, self).__setattr__(attr, value)


# SQL Generation.


class AliasManager(object):
    def __init__(self):
        # A list of dictionaries containing mappings at various depths.
        self._counter = 0
        self._current_index = 0
        self._mapping = []
        self.push()

    @property
    def mapping(self):
        return self._mapping[self._current_index - 1]

    def add(self, source):
        if source not in self.mapping:
            self._counter += 1
            self[source] = 't%d' % self._counter
        return self.mapping[source]

    def get(self, source, any_depth=False):
        if any_depth:
            for idx in reversed(range(self._current_index)):
                if source in self._mapping[idx]:
                    return self._mapping[idx][source]
        return self.add(source)

    def __getitem__(self, source):
        return self.get(source)

    def __setitem__(self, source, alias):
        self.mapping[source] = alias

    def push(self):
        self._current_index += 1
        if self._current_index > len(self._mapping):
            self._mapping.append({})

    def pop(self):
        if self._current_index == 1:
            raise ValueError('Cannot pop() from empty alias manager.')
        self._current_index -= 1


class State(collections.namedtuple('_State', ('scope', 'parentheses',
                                              'subquery', 'settings'))):
    def __new__(cls, scope=SCOPE_NORMAL, parentheses=False, subquery=False,
                **kwargs):
        return super(State, cls).__new__(cls, scope, parentheses, subquery,
                                         kwargs)

    def __call__(self, scope=None, parentheses=None, subquery=None, **kwargs):
        # All state is "inherited" except parentheses.
        scope = self.scope if scope is None else scope
        subquery = self.subquery if subquery is None else subquery

        # Try to avoid unnecessary dict copying.
        if kwargs and self.settings:
            settings = self.settings.copy()  # Copy original settings dict.
            settings.update(kwargs)  # Update copy with overrides.
        elif kwargs:
            settings = kwargs
        else:
            settings = self.settings
        return State(scope, parentheses, subquery, **settings)

    def __getattr__(self, attr_name):
        return self.settings.get(attr_name)


def __scope_context__(scope):
    @contextmanager
    def inner(self, **kwargs):
        with self(scope=scope, **kwargs):
            yield self
    return inner


class Context(object):
    def __init__(self, **settings):
        self.stack = []
        self._sql = []
        self._values = []
        self.alias_manager = AliasManager()
        self.state = State(**settings)

    def column_sort_key(self, item):
        return item[0].get_sort_key(self)

    @property
    def scope(self):
        return self.state.scope

    @property
    def parentheses(self):
        return self.state.parentheses

    @property
    def subquery(self):
        return self.state.subquery

    def __call__(self, **overrides):
        if overrides and overrides.get('scope') == self.scope:
            del overrides['scope']

        self.stack.append(self.state)
        self.state = self.state(**overrides)
        return self

    scope_normal = __scope_context__(SCOPE_NORMAL)
    scope_source = __scope_context__(SCOPE_SOURCE)
    scope_values = __scope_context__(SCOPE_VALUES)
    scope_cte = __scope_context__(SCOPE_CTE)
    scope_column = __scope_context__(SCOPE_COLUMN)

    def __enter__(self):
        if self.parentheses:
            self.literal('(')
        return self

    def __exit__(self, exc_type, exc_val, exc_tb):
        if self.parentheses:
            self.literal(')')
        self.state = self.stack.pop()

    @contextmanager
    def push_alias(self):
        self.alias_manager.push()
        yield
        self.alias_manager.pop()

    def sql(self, obj):
        if isinstance(obj, (Node, Context)):
            return obj.__sql__(self)
        elif is_model(obj):
            return obj._meta.table.__sql__(self)
        else:
            return self.sql(Value(obj))

    def literal(self, keyword):
        self._sql.append(keyword)
        return self

    def value(self, value, converter=None):
        if converter is None:
            converter = self.state.converter
        if converter:
            value = converter(value)
        self._values.append(value)
        return self

    def __sql__(self, ctx):
        ctx._sql.extend(self._sql)
        ctx._values.extend(self._values)
        return ctx

    def parse(self, node):
        return self.sql(node).query()

    def query(self):
        return ''.join(self._sql), self._values


# AST.


class Node(object):
    _coerce = True

    def clone(self):
        obj = self.__class__.__new__(self.__class__)
        obj.__dict__ = self.__dict__.copy()
        return obj

    def __sql__(self, ctx):
        raise NotImplementedError

    @staticmethod
    def copy(method):
        def inner(self, *args, **kwargs):
            clone = self.clone()
            method(clone, *args, **kwargs)
            return clone
        return inner

    def coerce(self, _coerce=True):
        if _coerce != self._coerce:
            clone = self.clone()
            clone._coerce = _coerce
            return clone
        return self

    def unwrap(self):
        return self


class ColumnFactory(object):
    __slots__ = ('node',)

    def __init__(self, node):
        self.node = node

    def __getattr__(self, attr):
        return Column(self.node, attr)


class _DynamicColumn(object):
    __slots__ = ()

    def __get__(self, instance, instance_type=None):
        if instance is not None:
            return ColumnFactory(instance)  # Implements __getattr__().
        return self


class _ExplicitColumn(object):
    __slots__ = ()

    def __get__(self, instance, instance_type=None):
        if instance is not None:
            raise AttributeError(
                '%s specifies columns explicitly, and does not support '
                'dynamic column lookups.' % instance)
        return self


class Source(Node):
    c = _DynamicColumn()

    def __init__(self, alias=None):
        super(Source, self).__init__()
        self._alias = alias

    @Node.copy
    def alias(self, name):
        self._alias = name

    def select(self, *columns):
        return Select((self,), columns)

    def join(self, dest, join_type='INNER', on=None):
        return Join(self, dest, join_type, on)

    def left_outer_join(self, dest, on=None):
        return Join(self, dest, JOIN.LEFT_OUTER, on)

    def get_sort_key(self, ctx):
        if self._alias:
            return (self._alias,)
        return (ctx.alias_manager[self],)

    def apply_alias(self, ctx):
        # If we are defining the source, include the "AS alias" declaration. An
        # alias is created for the source if one is not already defined.
        if ctx.scope == SCOPE_SOURCE:
            if self._alias:
                ctx.alias_manager[self] = self._alias
            ctx.literal(' AS ').sql(Entity(ctx.alias_manager[self]))
        return ctx

    def apply_column(self, ctx):
        if self._alias:
            ctx.alias_manager[self] = self._alias
        return ctx.sql(Entity(ctx.alias_manager[self]))


class _HashableSource(object):
    def __init__(self, *args, **kwargs):
        super(_HashableSource, self).__init__(*args, **kwargs)
        self._update_hash()

    @Node.copy
    def alias(self, name):
        self._alias = name
        self._update_hash()

    def _update_hash(self):
        self._hash = self._get_hash()

    def _get_hash(self):
        return hash((self.__class__, self._path, self._alias))

    def __hash__(self):
        return self._hash

    def __eq__(self, other):
        return self._hash == other._hash

    def __ne__(self, other):
        return not (self == other)


def __bind_database__(meth):
    @wraps(meth)
    def inner(self, *args, **kwargs):
        result = meth(self, *args, **kwargs)
        if self._database:
            return result.bind(self._database)
        return result
    return inner


def __join__(join_type='INNER', inverted=False):
    def method(self, other):
        if inverted:
            self, other = other, self
        return Join(self, other, join_type=join_type)
    return method


class BaseTable(Source):
    __and__ = __join__(JOIN.INNER)
    __add__ = __join__(JOIN.LEFT_OUTER)
    __sub__ = __join__(JOIN.RIGHT_OUTER)
    __or__ = __join__(JOIN.FULL_OUTER)
    __mul__ = __join__(JOIN.CROSS)
    __rand__ = __join__(JOIN.INNER, inverted=True)
    __radd__ = __join__(JOIN.LEFT_OUTER, inverted=True)
    __rsub__ = __join__(JOIN.RIGHT_OUTER, inverted=True)
    __ror__ = __join__(JOIN.FULL_OUTER, inverted=True)
    __rmul__ = __join__(JOIN.CROSS, inverted=True)


class _BoundTableContext(_callable_context_manager):
    def __init__(self, table, database):
        self.table = table
        self.database = database

    def __enter__(self):
        self._orig_database = self.table._database
        self.table.bind(self.database)
        if self.table._model is not None:
            self.table._model.bind(self.database)
        return self.table

    def __exit__(self, exc_type, exc_val, exc_tb):
        self.table.bind(self._orig_database)
        if self.table._model is not None:
            self.table._model.bind(self._orig_database)


class Table(_HashableSource, BaseTable):
    def __init__(self, name, columns=None, primary_key=None, schema=None,
                 alias=None, _model=None, _database=None):
        self.__name__ = name
        self._columns = columns
        self._primary_key = primary_key
        self._schema = schema
        self._path = (schema, name) if schema else (name,)
        self._model = _model
        self._database = _database
        super(Table, self).__init__(alias=alias)

        # Allow tables to restrict what columns are available.
        if columns is not None:
            self.c = _ExplicitColumn()
            for column in columns:
                setattr(self, column, Column(self, column))

        if primary_key:
            col_src = self if self._columns else self.c
            self.primary_key = getattr(col_src, primary_key)
        else:
            self.primary_key = None

    def clone(self):
        # Ensure a deep copy of the column instances.
        return Table(
            self.__name__,
            columns=self._columns,
            primary_key=self._primary_key,
            schema=self._schema,
            alias=self._alias,
            _model=self._model,
            _database=self._database)

    def bind(self, database=None):
        self._database = database
        return self

    def bind_ctx(self, database=None):
        return _BoundTableContext(self, database)

    def _get_hash(self):
        return hash((self.__class__, self._path, self._alias, self._model))

    @__bind_database__
    def select(self, *columns):
        if not columns and self._columns:
            columns = [Column(self, column) for column in self._columns]
        return Select((self,), columns)

    @__bind_database__
    def insert(self, insert=None, columns=None, **kwargs):
        if kwargs:
            insert = {} if insert is None else insert
            src = self if self._columns else self.c
            for key, value in kwargs.items():
                insert[getattr(src, key)] = value
        return Insert(self, insert=insert, columns=columns)

    @__bind_database__
    def replace(self, insert=None, columns=None, **kwargs):
        return (self
                .insert(insert=insert, columns=columns)
                .on_conflict('REPLACE'))

    @__bind_database__
    def update(self, update=None, **kwargs):
        if kwargs:
            update = {} if update is None else update
            for key, value in kwargs.items():
                src = self if self._columns else self.c
                update[getattr(self, key)] = value
        return Update(self, update=update)

    @__bind_database__
    def delete(self):
        return Delete(self)

    def __sql__(self, ctx):
        if ctx.scope == SCOPE_VALUES:
            # Return the quoted table name.
            return ctx.sql(Entity(*self._path))

        if self._alias:
            ctx.alias_manager[self] = self._alias

        if ctx.scope == SCOPE_SOURCE:
            # Define the table and its alias.
            return self.apply_alias(ctx.sql(Entity(*self._path)))
        else:
            # Refer to the table using the alias.
            return self.apply_column(ctx)


class Join(BaseTable):
    def __init__(self, lhs, rhs, join_type=JOIN.INNER, on=None, alias=None):
        super(Join, self).__init__(alias=alias)
        self.lhs = lhs
        self.rhs = rhs
        self.join_type = join_type
        self._on = on

    def on(self, predicate):
        self._on = predicate
        return self

    def __sql__(self, ctx):
        (ctx
         .sql(self.lhs)
         .literal(' %s JOIN ' % self.join_type)
         .sql(self.rhs))
        if self._on is not None:
            ctx.literal(' ON ').sql(self._on)
        return ctx


class CTE(_HashableSource, Source):
    def __init__(self, name, query, recursive=False, columns=None):
        self._alias = name
        self._nested_cte_list = query._cte_list
        query._cte_list = ()
        self._query = query
        self._recursive = recursive
        if columns is not None:
            columns = [Entity(c) if isinstance(c, basestring) else c
                       for c in columns]
        self._columns = columns
        super(CTE, self).__init__(alias=name)

    def _get_hash(self):
        return hash((self.__class__, self._alias, id(self._query)))

    def __sql__(self, ctx):
        if ctx.scope != SCOPE_CTE:
            return ctx.sql(Entity(self._alias))

        with ctx.push_alias():
            ctx.alias_manager[self] = self._alias
            ctx.sql(Entity(self._alias))

            if self._columns:
                ctx.literal(' ').sql(EnclosedNodeList(self._columns))
            ctx.literal(' AS (')
            with ctx.scope_normal():
                ctx.sql(self._query)
            ctx.literal(')')
        return ctx


class ColumnBase(Node):
    def alias(self, alias):
        if alias:
            return Alias(self, alias)
        return self

    def unalias(self):
        return self

    def cast(self, as_type):
        return Cast(self, as_type)

    def asc(self, collation=None, nulls=None):
        return Asc(self, collation=collation, nulls=nulls)
    __pos__ = asc

    def desc(self, collation=None, nulls=None):
        return Desc(self, collation=collation, nulls=nulls)
    __neg__ = desc

    def __invert__(self):
        return Negated(self)

    def _e(op, inv=False):
        """
        Lightweight factory which returns a method that builds an Expression
        consisting of the left-hand and right-hand operands, using `op`.
        """
        def inner(self, rhs):
            if inv:
                return Expression(rhs, op, self)
            return Expression(self, op, rhs)
        return inner
    __and__ = _e(OP.AND)
    __or__ = _e(OP.OR)

    __add__ = _e(OP.ADD)
    __sub__ = _e(OP.SUB)
    __mul__ = _e(OP.MUL)
    __div__ = __truediv__ = _e(OP.DIV)
    __xor__ = _e(OP.XOR)
    __radd__ = _e(OP.ADD, inv=True)
    __rsub__ = _e(OP.SUB, inv=True)
    __rmul__ = _e(OP.MUL, inv=True)
    __rdiv__ = __rtruediv__ = _e(OP.DIV, inv=True)
    __rand__ = _e(OP.AND, inv=True)
    __ror__ = _e(OP.OR, inv=True)
    __rxor__ = _e(OP.XOR, inv=True)

    def __eq__(self, rhs):
        op = OP.IS if rhs is None else OP.EQ
        return Expression(self, op, rhs)
    def __ne__(self, rhs):
        op = OP.IS_NOT if rhs is None else OP.NE
        return Expression(self, op, rhs)

    __lt__ = _e(OP.LT)
    __le__ = _e(OP.LTE)
    __gt__ = _e(OP.GT)
    __ge__ = _e(OP.GTE)
    __lshift__ = _e(OP.IN)
    __rshift__ = _e(OP.IS)
    __mod__ = _e(OP.LIKE)
    __pow__ = _e(OP.ILIKE)

    bin_and = _e(OP.BIN_AND)
    bin_or = _e(OP.BIN_OR)
    in_ = _e(OP.IN)
    not_in = _e(OP.NOT_IN)
    regexp = _e(OP.REGEXP)

    # Special expressions.
    def is_null(self, is_null=True):
        op = OP.IS if is_null else OP.IS_NOT
        return Expression(self, op, None)
    def contains(self, rhs):
        return Expression(self, OP.ILIKE, '%%%s%%' % rhs)
    def startswith(self, rhs):
        return Expression(self, OP.ILIKE, '%s%%' % rhs)
    def endswith(self, rhs):
        return Expression(self, OP.ILIKE, '%%%s' % rhs)
    def between(self, lo, hi):
        return Expression(self, OP.BETWEEN, NodeList((lo, SQL('AND'), hi)))
    def concat(self, rhs):
        return StringExpression(self, OP.CONCAT, rhs)
    def __getitem__(self, item):
        if isinstance(item, slice):
            if item.start is None or item.stop is None:
                raise ValueError('BETWEEN range must have both a start- and '
                                 'end-point.')
            return self.between(item.start, item.stop)
        return self == item

    def distinct(self):
        return NodeList((SQL('DISTINCT'), self))

    def get_sort_key(self, ctx):
        return ()


class Column(ColumnBase):
    def __init__(self, source, name):
        self.source = source
        self.name = name

    def get_sort_key(self, ctx):
        if ctx.scope == SCOPE_VALUES:
            return (self.name,)
        else:
            return self.source.get_sort_key(ctx) + (self.name,)

    def __hash__(self):
        return hash((self.source, self.name))

    def __sql__(self, ctx):
        if ctx.scope == SCOPE_VALUES:
            return ctx.sql(Entity(self.name))
        else:
            with ctx.scope_column():
                return ctx.sql(self.source).literal('.').sql(Entity(self.name))


class WrappedNode(ColumnBase):
    def __init__(self, node):
        self.node = node
        self._coerce = getattr(node, '_coerce', True)

    def unwrap(self):
        return self.node.unwrap()


class EntityFactory(object):
    __slots__ = ('node',)
    def __init__(self, node):
        self.node = node
    def __getattr__(self, attr):
        return Entity(self.node, attr)


class _DynamicEntity(object):
    __slots__ = ()
    def __get__(self, instance, instance_type=None):
        if instance is not None:
            return EntityFactory(instance._alias)  # Implements __getattr__().
        return self


class Alias(WrappedNode):
    c = _DynamicEntity()

    def __init__(self, node, alias):
        super(Alias, self).__init__(node)
        self._alias = alias

    def alias(self, alias=None):
        if alias is None:
            return self.node
        else:
            return Alias(self.node, alias)

    def unalias(self):
        return self.node

    def __sql__(self, ctx):
        if ctx.scope == SCOPE_SOURCE:
            return (ctx
                    .sql(self.node)
                    .literal(' AS ')
                    .sql(Entity(self._alias)))
        else:
            return ctx.sql(Entity(self._alias))


class Negated(WrappedNode):
    def __invert__(self):
        return self.node

    def __sql__(self, ctx):
        return ctx.literal('NOT ').sql(self.node)


class BitwiseMixin(object):
    def __and__(self, other):
        return self.bin_and(other)

    def __or__(self, other):
        return self.bin_or(other)

    def __sub__(self, other):
        return self.bin_and(other.bin_negated())

    def __invert__(self):
        return BitwiseNegated(self)


class BitwiseNegated(BitwiseMixin, WrappedNode):
    def __invert__(self):
        return self.node

    def __sql__(self, ctx):
        if ctx.state.operations:
            op_sql = ctx.state.operations.get(self.op, self.op)
        else:
            op_sql = self.op
        return ctx.literal(op_sql).sql(self.node)


class Value(ColumnBase):
    def __init__(self, value, converter=None, unpack=True):
        self.value = value
        self.converter = converter
        self.multi = isinstance(self.value, (list, set, tuple)) and unpack
        if self.multi:
            self.values = []
            for item in self.value:
                if isinstance(item, Node):
                    self.values.append(item)
                else:
                    self.values.append(Value(item, self.converter))

    def __sql__(self, ctx):
        if self.multi:
            ctx.sql(EnclosedNodeList(self.values))
        else:
            (ctx
             .literal(ctx.state.param or '?')
             .value(self.value, self.converter))
        return ctx


def AsIs(value):
    return Value(value, unpack=False)


class Cast(WrappedNode):
    def __init__(self, node, cast):
        super(Cast, self).__init__(node)
        self.cast = cast
        self._coerce = False

    def __sql__(self, ctx):
        return (ctx
                .literal('CAST(')
                .sql(self.node)
                .literal(' AS %s)' % self.cast))


class Ordering(WrappedNode):
    def __init__(self, node, direction, collation=None, nulls=None):
        super(Ordering, self).__init__(node)
        self.direction = direction
        self.collation = collation
        self.nulls = nulls

    def collate(self, collation=None):
        return Ordering(self.node, self.direction, collation)

    def __sql__(self, ctx):
        ctx.sql(self.node).literal(' %s' % self.direction)
        if self.collation:
            ctx.literal(' COLLATE %s' % self.collation)
        if self.nulls:
            ctx.literal(' NULLS %s' % self.nulls)
        return ctx


def Asc(node, collation=None, nulls=None):
    return Ordering(node, 'ASC', collation, nulls)


def Desc(node, collation=None, nulls=None):
    return Ordering(node, 'DESC', collation, nulls)


class Expression(ColumnBase):
    def __init__(self, lhs, op, rhs, flat=False):
        self.lhs = lhs
        self.op = op
        self.rhs = rhs
        self.flat = flat

    def __sql__(self, ctx):
        overrides = {'parentheses': not self.flat}
        if isinstance(self.lhs, Field):
            overrides['converter'] = self.lhs.db_value
        else:
            overrides['converter'] = None

        if ctx.state.operations:
            op_sql = ctx.state.operations.get(self.op, self.op)
        else:
            op_sql = self.op

        with ctx(**overrides):
            # Postgresql reports an error for IN/NOT IN (), so convert to
            # the equivalent boolean expression.
            if self.op == OP.IN and Context().parse(self.rhs)[0] == '()':
                return ctx.literal('0 = 1')
            elif self.op == OP.NOT_IN and Context().parse(self.rhs)[0] == '()':
                return ctx.literal('1 = 1')

            return (ctx
                    .sql(self.lhs)
                    .literal(' %s ' % op_sql)
                    .sql(self.rhs))


class StringExpression(Expression):
    def __add__(self, rhs):
        return self.concat(rhs)
    def __radd__(self, lhs):
        return StringExpression(lhs, OP.CONCAT, self)


class Entity(ColumnBase):
    def __init__(self, *path):
        self._path = [part.replace('"', '""') for part in path if part]

    def __getattr__(self, attr):
        return Entity(*self._path + [attr])

    def get_sort_key(self, ctx):
        return tuple(self._path)

    def __hash__(self):
        return hash((self.__class__.__name__, tuple(self._path)))

    def __sql__(self, ctx):
        return ctx.literal(quote(self._path, ctx.state.quote or '"'))


class SQL(ColumnBase):
    def __init__(self, sql, params=None):
        self.sql = sql
        self.params = params

    def __sql__(self, ctx):
        ctx.literal(self.sql)
        if self.params:
            for param in self.params:
                if isinstance(param, Node):
                    ctx.sql(param)
                else:
                    ctx.value(param)
        return ctx


def Check(constraint):
    return SQL('CHECK (%s)' % constraint)


class Function(ColumnBase):
    def __init__(self, name, arguments, coerce=True):
        self.name = name
        self.arguments = arguments
        if name and name.lower() in ('sum', 'count', 'cast'):
            self._coerce = False
        else:
            self._coerce = coerce

    def __getattr__(self, attr):
        def decorator(*args, **kwargs):
            return Function(attr, args, **kwargs)
        return decorator

    def over(self, partition_by=None, order_by=None, start=None, end=None,
             window=None):
        if isinstance(partition_by, Window) and window is None:
            window = partition_by
        if start is not None and not isinstance(start, SQL):
            start = SQL(*start)
        if end is not None and not isinstance(end, SQL):
            end = SQL(*end)

        if window is None:
            node = Window(partition_by=partition_by, order_by=order_by,
                          start=start, end=end)
        else:
            node = SQL(window._alias)
        return NodeList((self, SQL('OVER'), node))

    def __sql__(self, ctx):
        ctx.literal(self.name)
        if not len(self.arguments):
            ctx.literal('()')
        else:
            # Special-case to avoid double-wrapping functions whose only
            # argument is a sub-query.
            if len(self.arguments) == 1 and isinstance(self.arguments[0],
                                                       SelectQuery):
                wrapper = CommaNodeList
            else:
                wrapper = EnclosedNodeList
            ctx.sql(wrapper([
                (argument if isinstance(argument, Node)
                 else Value(argument))
                for argument in self.arguments]))
        return ctx


fn = Function(None, None)


class Window(Node):
    CURRENT_ROW = 'CURRENT ROW'

    def __init__(self, partition_by=None, order_by=None, start=None, end=None,
                 alias=None):
        super(Window, self).__init__()
        self.partition_by = partition_by
        self.order_by = order_by
        self.start = start
        self.end = end
        if self.start is None and self.end is not None:
            raise ValueError('Cannot specify WINDOW end without start.')
        self._alias = alias or 'w'

    def alias(self, alias=None):
        self._alias = alias or 'w'
        return self

    @staticmethod
    def following(value=None):
        if value is None:
            return SQL('UNBOUNDED FOLLOWING')
        return SQL('%d FOLLOWING' % value)

    @staticmethod
    def preceding(value=None):
        if value is None:
            return SQL('UNBOUNDED PRECEDING')
        return SQL('%d PRECEDING' % value)

    def __sql__(self, ctx):
        if ctx.scope != SCOPE_SOURCE:
            ctx.literal(self._alias)
            ctx.literal(' AS ')

        with ctx(parentheses=True):
            parts = []
            if self.partition_by:
                parts.extend((
                    SQL('PARTITION BY'),
                    CommaNodeList(self.partition_by)))
            if self.order_by:
                parts.extend((
                    SQL('ORDER BY'),
                    CommaNodeList(self.order_by)))
            if self.start is not None and self.end is not None:
                parts.extend((
                    SQL('ROWS BETWEEN'),
                    self.start,
                    SQL('AND'),
                    self.end))
            elif self.start is not None:
                parts.extend((SQL('ROWS'), self.start))
            ctx.sql(NodeList(parts))
        return ctx

    def clone_base(self):
        return Window(self.partition_by, self.order_by)


def Case(predicate, expression_tuples, default=None):
    clauses = [SQL('CASE')]
    if predicate is not None:
        clauses.append(predicate)
    for expr, value in expression_tuples:
        clauses.extend((SQL('WHEN'), expr, SQL('THEN'), value))
    if default is not None:
        clauses.extend((SQL('ELSE'), default))
    clauses.append(SQL('END'))
    return NodeList(clauses)


class NodeList(ColumnBase):
    def __init__(self, nodes, glue=' ', parens=False):
        self.nodes = nodes
        self.glue = glue
        self.parens = parens
        if parens and len(self.nodes) == 1:
            if isinstance(self.nodes[0], Expression):
                # Hack to avoid double-parentheses.
                self.nodes[0].flat = True

    def __sql__(self, ctx):
        n_nodes = len(self.nodes)
        if n_nodes == 0:
            return ctx.literal('()') if self.parens else ctx
        with ctx(parentheses=self.parens):
            for i in range(n_nodes - 1):
                ctx.sql(self.nodes[i])
                ctx.literal(self.glue)
            ctx.sql(self.nodes[n_nodes - 1])
        return ctx


def CommaNodeList(nodes):
    return NodeList(nodes, ', ')


def EnclosedNodeList(nodes):
    return NodeList(nodes, ', ', True)


class DQ(ColumnBase):
    def __init__(self, **query):
        super(DQ, self).__init__()
        self.query = query
        self._negated = False

    @Node.copy
    def __invert__(self):
        self._negated = not self._negated

    def clone(self):
        node = DQ(**self.query)
        node._negated = self._negated
        return node

#: Represent a row tuple.
Tuple = lambda *a: EnclosedNodeList(a)


class QualifiedNames(WrappedNode):
    def __sql__(self, ctx):
        with ctx.scope_column():
            return ctx.sql(self.node)


class OnConflict(Node):
    def __init__(self, action=None, update=None, preserve=None, where=None,
                 conflict_target=None):
        self._action = action
        self._update = update
        self._preserve = ensure_tuple(preserve)
        self._where = where
        self._conflict_target = ensure_tuple(conflict_target)

    def get_conflict_statement(self, ctx):
        return ctx.state.conflict_statement(self)

    def get_conflict_update(self, ctx):
        return ctx.state.conflict_update(self)

    @Node.copy
    def preserve(self, *columns):
        self._preserve = columns

    @Node.copy
    def update(self, _data=None, **kwargs):
        if _data and kwargs and not isinstance(_data, dict):
            raise ValueError('Cannot mix data with keyword arguments in the '
                             'OnConflict update method.')
        _data = _data or {}
        if kwargs:
            _data.update(kwargs)
        self._update = _data

    @Node.copy
    def where(self, *expressions):
        if self._where is not None:
            expressions = (self._where,) + expressions
        self._where = reduce(operator.and_, expressions)

    @Node.copy
    def conflict_target(self, *constraints):
        self._conflict_target = constraints


def database_required(method):
    @wraps(method)
    def inner(self, database=None, *args, **kwargs):
        database = self._database if database is None else database
        if not database:
            raise Exception('Query must be bound to a database in order '
                            'to call "%s".' % method.__name__)
        return method(self, database, *args, **kwargs)
    return inner

# BASE QUERY INTERFACE.

class BaseQuery(Node):
    default_row_type = ROW.DICT

    def __init__(self, _database=None, **kwargs):
        self._database = _database
        self._cursor_wrapper = None
        self._row_type = None
        self._constructor = None
        super(BaseQuery, self).__init__(**kwargs)

    def bind(self, database=None):
        self._database = database
        return self

    def clone(self):
        query = super(BaseQuery, self).clone()
        query._cursor_wrapper = None
        return query

    @Node.copy
    def dicts(self, as_dict=True):
        self._row_type = ROW.DICT if as_dict else None
        return self

    @Node.copy
    def tuples(self, as_tuple=True):
        self._row_type = ROW.TUPLE if as_tuple else None
        return self

    @Node.copy
    def namedtuples(self, as_namedtuple=True):
        self._row_type = ROW.NAMED_TUPLE if as_namedtuple else None
        return self

    @Node.copy
    def objects(self, constructor=None):
        self._row_type = ROW.CONSTRUCTOR if constructor else None
        self._constructor = constructor
        return self

    def _get_cursor_wrapper(self, cursor):
        row_type = self._row_type or self.default_row_type

        if row_type == ROW.DICT:
            return DictCursorWrapper(cursor)
        elif row_type == ROW.TUPLE:
            return CursorWrapper(cursor)
        elif row_type == ROW.NAMED_TUPLE:
            return NamedTupleCursorWrapper(cursor)
        elif row_type == ROW.CONSTRUCTOR:
            return ObjectCursorWrapper(cursor, self._constructor)
        else:
            raise ValueError('Unrecognized row type: "%s".' % row_type)

    def __sql__(self, ctx):
        raise NotImplementedError

    def sql(self):
        if self._database:
            context = self._database.get_sql_context()
        else:
            context = Context()
        return context.parse(self)

    @database_required
    def execute(self, database):
        return self._execute(database)

    def _execute(self, database):
        raise NotImplementedError

    def iterator(self, database=None):
        return iter(self.execute(database).iterator())

    def _ensure_execution(self):
        if not self._cursor_wrapper:
            if not self._database:
                raise ValueError('Query has not been executed.')
            self.execute()

    def __iter__(self):
        self._ensure_execution()
        return iter(self._cursor_wrapper)

    def __getitem__(self, value):
        self._ensure_execution()
        if isinstance(value, slice):
            index = value.stop
        else:
            index = value
        if index is not None and index >= 0:
            index += 1
        self._cursor_wrapper.fill_cache(index)
        return self._cursor_wrapper.row_cache[value]

    def __len__(self):
        self._ensure_execution()
        return len(self._cursor_wrapper)


class RawQuery(BaseQuery):
    def __init__(self, sql=None, params=None, **kwargs):
        super(RawQuery, self).__init__(**kwargs)
        self._sql = sql
        self._params = params

    def __sql__(self, ctx):
        ctx.literal(self._sql)
        if self._params:
            for param in self._params:
                if isinstance(param, Node):
                    ctx.sql(param)
                else:
                    ctx.value(param)
        return ctx

    def _execute(self, database):
        if self._cursor_wrapper is None:
            cursor = database.execute(self)
            self._cursor_wrapper = self._get_cursor_wrapper(cursor)
        return self._cursor_wrapper


class Query(BaseQuery):
    def __init__(self, where=None, order_by=None, limit=None, offset=None,
                 **kwargs):
        super(Query, self).__init__(**kwargs)
        self._where = where
        self._order_by = order_by
        self._limit = limit
        self._offset = offset

        self._cte_list = None

    @Node.copy
    def with_cte(self, *cte_list):
        self._cte_list = cte_list

    @Node.copy
    def where(self, *expressions):
        if self._where is not None:
            expressions = (self._where,) + expressions
        self._where = reduce(operator.and_, expressions)

    @Node.copy
    def order_by(self, *values):
        self._order_by = values

    @Node.copy
    def order_by_extend(self, *values):
        self._order_by = ((self._order_by or ()) + values) or None

    @Node.copy
    def limit(self, value=None):
        self._limit = value

    @Node.copy
    def offset(self, value=None):
        self._offset = value

    @Node.copy
    def paginate(self, page, paginate_by=20):
        if page > 0:
            page -= 1
        self._limit = paginate_by
        self._offset = page * paginate_by

    def _apply_ordering(self, ctx):
        if self._order_by:
            (ctx
             .literal(' ORDER BY ')
             .sql(CommaNodeList(self._order_by)))
        if self._limit is not None or (self._offset is not None and
                                       ctx.state.limit_max):
            ctx.literal(' LIMIT %d' % (self._limit or ctx.state.limit_max))
        if self._offset is not None:
            ctx.literal(' OFFSET %d' % self._offset)
        return ctx

    def __sql__(self, ctx):
        if self._cte_list:
            # The CTE scope is only used at the very beginning of the query,
            # when we are describing the various CTEs we will be using.
            recursive = any(cte._recursive for cte in self._cte_list)
            with ctx.scope_cte():
                (ctx
                 .literal('WITH RECURSIVE ' if recursive else 'WITH ')
                 .sql(CommaNodeList(self._cte_list))
                 .literal(' '))
        return ctx


def __compound_select__(operation, inverted=False):
    def method(self, other):
        if inverted:
            self, other = other, self
        return CompoundSelectQuery(self, operation, other)
    return method


class SelectQuery(Query):
    __add__ = __compound_select__('UNION ALL')
    __or__ = __compound_select__('UNION')
    __and__ = __compound_select__('INTERSECT')
    __sub__ = __compound_select__('EXCEPT')
    __radd__ = __compound_select__('UNION ALL', inverted=True)
    __ror__ = __compound_select__('UNION', inverted=True)
    __rand__ = __compound_select__('INTERSECT', inverted=True)
    __rsub__ = __compound_select__('EXCEPT', inverted=True)

    def cte(self, name, recursive=False, columns=None):
        return CTE(name, self, recursive=recursive, columns=columns)


class SelectBase(_HashableSource, Source, SelectQuery):
    def _get_hash(self):
        return hash((self.__class__, self._alias or id(self)))

    def _execute(self, database):
        if self._cursor_wrapper is None:
            cursor = database.execute(self)
            self._cursor_wrapper = self._get_cursor_wrapper(cursor)
        return self._cursor_wrapper

    @database_required
    def peek(self, database, n=1):
        rows = self.execute(database)[:n]
        if rows:
            return rows[0] if n == 1 else rows

    @database_required
    def first(self, database, n=1):
        if self._limit != n:
            self._limit = n
            self._cursor_wrapper = None
        return self.peek(database, n=n)

    @database_required
    def scalar(self, database, as_tuple=False):
        row = self.tuples().peek(database)
        return row[0] if row and not as_tuple else row

    @database_required
    def count(self, database, clear_limit=False):
        clone = self.order_by().alias('_wrapped')
        if clear_limit:
            clone._limit = clone._offset = None
        try:
            if clone._having is None and clone._windows is None and \
               clone._distinct is None and clone._simple_distinct is not True:
                clone = clone.select(SQL('1'))
        except AttributeError:
            pass
        return Select([clone], [fn.COUNT(SQL('1'))]).scalar(database)

    @database_required
    def exists(self, database):
        clone = self.columns(SQL('1'))
        clone._limit = 1
        clone._offset = None
        return bool(clone.scalar())

    @database_required
    def get(self, database):
        self._cursor_wrapper = None
        try:
            return self.execute(database)[0]
        except IndexError:
            pass


# QUERY IMPLEMENTATIONS.


class CompoundSelectQuery(SelectBase):
    def __init__(self, lhs, op, rhs):
        super(CompoundSelectQuery, self).__init__()
        self.lhs = lhs
        self.op = op
        self.rhs = rhs

    @property
    def _returning(self):
        return self.lhs._returning

    def _get_query_key(self):
        return (self.lhs.get_query_key(), self.rhs.get_query_key())

    def __sql__(self, ctx):
        if ctx.scope == SCOPE_COLUMN:
            return self.apply_column(ctx)

        parens_around_query = ctx.state.compound_select_parentheses
        outer_parens = ctx.subquery or (ctx.scope == SCOPE_SOURCE)
        with ctx(parentheses=outer_parens):
            with ctx.scope_normal(parentheses=parens_around_query,
                                  subquery=False):
                ctx.sql(self.lhs)
            ctx.literal(' %s ' % self.op)
            with ctx.push_alias():
                with ctx.scope_normal(parentheses=parens_around_query,
                                      subquery=False):
                    ctx.sql(self.rhs)

            # Apply ORDER BY, LIMIT, OFFSET.
            self._apply_ordering(ctx)

        return self.apply_alias(ctx)


class Select(SelectBase):
    def __init__(self, from_list=None, columns=None, group_by=None,
                 having=None, distinct=None, windows=None, for_update=None,
                 **kwargs):
        super(Select, self).__init__(**kwargs)
        self._from_list = (list(from_list) if isinstance(from_list, tuple)
                           else from_list) or []
        self._returning = columns
        self._group_by = group_by
        self._having = having
        self._windows = None
        self._for_update = 'FOR UPDATE' if for_update is True else for_update

        self._distinct = self._simple_distinct = None
        if distinct:
            if isinstance(distinct, bool):
                self._simple_distinct = distinct
            else:
                self._distinct = distinct

        self._cursor_wrapper = None

    @Node.copy
    def columns(self, *columns, **kwargs):
        self._returning = columns
    select = columns

    @Node.copy
    def select_extend(self, *columns):
        self._returning = tuple(self._returning) + columns

    @Node.copy
    def from_(self, *sources):
        self._from_list = list(sources)

    @Node.copy
    def join(self, dest, join_type='INNER', on=None):
        if not self._from_list:
            raise ValueError('No sources to join on.')
        item = self._from_list.pop()
        self._from_list.append(Join(item, dest, join_type, on))

    @Node.copy
    def group_by(self, *columns):
        grouping = []
        for column in columns:
            if isinstance(column, Table):
                if not column._columns:
                    raise ValueError('Cannot pass a table to group_by() that '
                                     'does not have columns explicitly '
                                     'declared.')
                grouping.extend([getattr(column, col_name)
                                 for col_name in column._columns])
            else:
                grouping.append(column)
        self._group_by = grouping

    @Node.copy
    def group_by_extend(self, *values):
        group_by = tuple(self._group_by or ()) + values
        return self.group_by(*group_by)

    @Node.copy
    def having(self, *expressions):
        if self._having is not None:
            expressions = (self._having,) + expressions
        self._having = reduce(operator.and_, expressions)

    @Node.copy
    def distinct(self, *columns):
        if len(columns) == 1 and (columns[0] is True or columns[0] is False):
            self._simple_distinct = columns[0]
        else:
            self._simple_distinct = False
            self._distinct = columns

    @Node.copy
    def window(self, *windows):
        self._windows = windows if windows else None

    @Node.copy
    def for_update(self, for_update=True):
        self._for_update = 'FOR UPDATE' if for_update is True else for_update

    def _get_query_key(self):
        return self._alias

    def __sql_selection__(self, ctx, is_subquery=False):
        return ctx.sql(CommaNodeList(self._returning))

    def __sql__(self, ctx):
        super(Select, self).__sql__(ctx)
        if ctx.scope == SCOPE_COLUMN:
            return self.apply_column(ctx)

        is_subquery = ctx.subquery
        parentheses = is_subquery or (ctx.scope == SCOPE_SOURCE)

        with ctx.scope_normal(converter=None, parentheses=parentheses,
                              subquery=True):
            ctx.literal('SELECT ')
            if self._simple_distinct or self._distinct is not None:
                ctx.literal('DISTINCT ')
                if self._distinct:
                    (ctx
                     .literal('ON ')
                     .sql(EnclosedNodeList(self._distinct))
                     .literal(' '))

            with ctx.scope_source():
                ctx = self.__sql_selection__(ctx, is_subquery)

            if self._from_list:
                with ctx.scope_source(parentheses=False):
                    ctx.literal(' FROM ').sql(CommaNodeList(self._from_list))

            if self._where is not None:
                ctx.literal(' WHERE ').sql(self._where)

            if self._group_by:
                ctx.literal(' GROUP BY ').sql(CommaNodeList(self._group_by))

            if self._having is not None:
                ctx.literal(' HAVING ').sql(self._having)

            if self._windows is not None:
                ctx.literal(' WINDOW ')
                ctx.sql(CommaNodeList(self._windows))

            # Apply ORDER BY, LIMIT, OFFSET.
            self._apply_ordering(ctx)

            if self._for_update:
                if not ctx.state.for_update:
                    raise ValueError('FOR UPDATE specified but not supported '
                                     'by database.')
                ctx.literal(' ')
                ctx.sql(SQL(self._for_update))

        ctx = self.apply_alias(ctx)
        return ctx


class _WriteQuery(Query):
    def __init__(self, table, returning=None, **kwargs):
        self.table = table
        self._returning = returning
        self._return_cursor = True if returning else False
        super(_WriteQuery, self).__init__(**kwargs)

    @Node.copy
    def returning(self, *returning):
        self._returning = returning
        self._return_cursor = True if returning else False

    def apply_returning(self, ctx):
        if self._returning:
            ctx.literal(' RETURNING ').sql(CommaNodeList(self._returning))
        return ctx

    def _execute(self, database):
        if self._returning:
            cursor = self.execute_returning(database)
        else:
            cursor = database.execute(self)
        return self.handle_result(database, cursor)

    def execute_returning(self, database):
        if self._cursor_wrapper is None:
            cursor = database.execute(self)
            self._cursor_wrapper = self._get_cursor_wrapper(cursor)
        return self._cursor_wrapper

    def handle_result(self, database, cursor):
        if self._return_cursor:
            return cursor
        return database.rows_affected(cursor)

    def _set_table_alias(self, ctx):
        ctx.alias_manager[self.table] = self.table.__name__

    def __sql__(self, ctx):
        super(_WriteQuery, self).__sql__(ctx)
        # We explicitly set the table alias to the table's name, which ensures
        # that if a sub-select references a column on the outer table, we won't
        # assign it a new alias (e.g. t2) but will refer to it as table.column.
        self._set_table_alias(ctx)
        return ctx


class Update(_WriteQuery):
    def __init__(self, table, update=None, **kwargs):
        super(Update, self).__init__(table, **kwargs)
        self._update = update
        self._from = None

    @Node.copy
    def from_(self, *sources):
        self._from = sources

    def __sql__(self, ctx):
        super(Update, self).__sql__(ctx)

        with ctx.scope_values(subquery=True):
            ctx.literal('UPDATE ')

            expressions = []
            for k, v in sorted(self._update.items(), key=ctx.column_sort_key):
                if not isinstance(v, Node):
                    converter = k.db_value if isinstance(k, Field) else None
                    v = Value(v, converter=converter, unpack=False)
                expressions.append(NodeList((k, SQL('='), v)))

            (ctx
             .sql(self.table)
             .literal(' SET ')
             .sql(CommaNodeList(expressions)))

            if self._from:
                ctx.literal(' FROM ').sql(CommaNodeList(self._from))

            if self._where:
                ctx.literal(' WHERE ').sql(self._where)
            self._apply_ordering(ctx)
            return self.apply_returning(ctx)


class Insert(_WriteQuery):
    SIMPLE = 0
    QUERY = 1
    MULTI = 2
    class DefaultValuesException(Exception): pass

    def __init__(self, table, insert=None, columns=None, on_conflict=None,
                 **kwargs):
        super(Insert, self).__init__(table, **kwargs)
        self._insert = insert
        self._columns = columns
        self._on_conflict = on_conflict
        self._query_type = None

    def where(self, *expressions):
        raise NotImplementedError('INSERT queries cannot have a WHERE clause.')

    @Node.copy
    def on_conflict_ignore(self, ignore=True):
        self._on_conflict = OnConflict('IGNORE') if ignore else None

    @Node.copy
    def on_conflict_replace(self, replace=True):
        self._on_conflict = OnConflict('REPLACE') if replace else None

    @Node.copy
    def on_conflict(self, *args, **kwargs):
        self._on_conflict = (OnConflict(*args, **kwargs) if (args or kwargs)
                             else None)

    def _simple_insert(self, ctx):
        if not self._insert:
            raise self.DefaultValuesException('Error: no data to insert.')
        return self._generate_insert((self._insert,), ctx)

    def get_default_data(self):
        return {}

    def _generate_insert(self, insert, ctx):
        rows_iter = iter(insert)
        columns = self._columns

        # Load and organize column defaults (if provided).
        defaults = self.get_default_data()

        if not columns:
            uses_strings = False
            try:
                row = next(rows_iter)
            except StopIteration:
                raise self.DefaultValuesException('Error: no rows to insert.')
            else:
                accum = []
                value_lookups = {}
                for key in row:
                    if isinstance(key, basestring):
                        column = getattr(self.table, key)
                        uses_strings = True
                    else:
                        column = key
                    accum.append(column)
                    value_lookups[column] = key

            column_set = set(accum)
            for column in (set(defaults) - column_set):
                accum.append(column)
                value_lookups[column] = column.name if uses_strings else column

            columns = sorted(accum, key=lambda obj: obj.get_sort_key(ctx))
            rows_iter = itertools.chain(iter((row,)), rows_iter)
        else:
            columns = list(columns)
            value_lookups = dict((column, column) for column in columns)
            for col in sorted(defaults, key=lambda obj: obj.get_sort_key(ctx)):
                if col not in value_lookups:
                    columns.append(col)
                    value_lookups[col] = col

        ctx.sql(EnclosedNodeList(columns)).literal(' VALUES ')
        columns_converters = [
            (column, column.db_value if isinstance(column, Field) else None)
            for column in columns]

        all_values = []
        for row in rows_iter:
            values = []
            is_dict = isinstance(row, collections.Mapping)
            for i, (column, converter) in enumerate(columns_converters):
                try:
                    if is_dict:
                        val = row[value_lookups[column]]
                    else:
                        val = row[i]
                except (KeyError, IndexError):
                    if column in defaults:
                        val = defaults[column]
                        if callable(val):
                            val = val()
                    else:
                        raise ValueError('Missing value for "%s".' % column)

                if not isinstance(val, Node):
                    val = Value(val, converter=converter, unpack=False)
                values.append(val)

            all_values.append(EnclosedNodeList(values))

        with ctx.scope_values(subquery=True):
            return ctx.sql(CommaNodeList(all_values))

    def _query_insert(self, ctx):
        return (ctx
                .sql(EnclosedNodeList(self._columns))
                .literal(' ')
                .sql(self._insert))

    def _default_values(self, ctx):
        if not self._database:
            return ctx.literal('DEFAULT VALUES')
        return self._database.default_values_insert(ctx)

    def __sql__(self, ctx):
        super(Insert, self).__sql__(ctx)
        with ctx.scope_values():
            statement = None
            if self._on_conflict is not None:
                statement = self._on_conflict.get_conflict_statement(ctx)

            (ctx
             .sql(statement or SQL('INSERT'))
             .literal(' INTO ')
             .sql(self.table)
             .literal(' '))

            if isinstance(self._insert, dict) and not self._columns:
                try:
                    self._simple_insert(ctx)
                except self.DefaultValuesException:
                    self._default_values(ctx)
                self._query_type = Insert.SIMPLE
            elif isinstance(self._insert, SelectQuery):
                self._query_insert(ctx)
                self._query_type = Insert.QUERY
            else:
                try:
                    self._generate_insert(self._insert, ctx)
                except self.DefaultValuesException:
                    return
                self._query_type = Insert.MULTI

            if self._on_conflict is not None:
                update = self._on_conflict.get_conflict_update(ctx)
                if update is not None:
                    ctx.literal(' ').sql(update)

            return self.apply_returning(ctx)

    def _execute(self, database):
        if self._returning is None and database.returning_clause \
           and self.table._primary_key:
            self._returning = (self.table._primary_key,)
        return super(Insert, self)._execute(database)

    def handle_result(self, database, cursor):
        if self._return_cursor:
            return cursor
        return database.last_insert_id(cursor, self._query_type)


class Delete(_WriteQuery):
    def __sql__(self, ctx):
        super(Delete, self).__sql__(ctx)

        with ctx.scope_values(subquery=True):
            ctx.literal('DELETE FROM ').sql(self.table)
            if self._where is not None:
                ctx.literal(' WHERE ').sql(self._where)

            self._apply_ordering(ctx)
            return self.apply_returning(ctx)


class Index(Node):
    def __init__(self, name, table, expressions, unique=False, safe=False,
                 where=None, using=None):
        self._name = name
        self._table = Entity(table) if not isinstance(table, Table) else table
        self._expressions = expressions
        self._where = where
        self._unique = unique
        self._safe = safe
        self._using = using

    @Node.copy
    def safe(self, _safe=True):
        self._safe = _safe

    @Node.copy
    def where(self, *expressions):
        if self._where is not None:
            expressions = (self._where,) + expressions
        self._where = reduce(operator.and_, expressions)

    @Node.copy
    def using(self, _using=None):
        self._using = _using

    def __sql__(self, ctx):
        statement = 'CREATE UNIQUE INDEX ' if self._unique else 'CREATE INDEX '
        with ctx.scope_values(subquery=True):
            ctx.literal(statement)
            if self._safe:
                ctx.literal('IF NOT EXISTS ')
            (ctx
             .sql(Entity(self._name))
             .literal(' ON ')
             .sql(self._table)
             .literal(' '))
            if self._using is not None:
                ctx.literal('USING %s ' % self._using)

            ctx.sql(EnclosedNodeList([
                SQL(expr) if isinstance(expr, basestring) else expr
                for expr in self._expressions]))
            if self._where is not None:
                ctx.literal(' WHERE ').sql(self._where)

        return ctx


class ModelIndex(Index):
    def __init__(self, model, fields, unique=False, safe=True, where=None,
                 using=None, name=None):
        self._model = model
        if name is None:
            name = self._generate_name_from_fields(model, fields)
        if using is None:
            for field in fields:
                if getattr(field, 'index_type', None):
                    using = field.index_type
        super(ModelIndex, self).__init__(
            name=name,
            table=model._meta.table,
            expressions=fields,
            unique=unique,
            safe=safe,
            where=where,
            using=using)

    def _generate_name_from_fields(self, model, fields):
        accum = []
        for field in fields:
            if isinstance(field, basestring):
                accum.append(field.split()[0])
            else:
                if isinstance(field, Node) and not isinstance(field, Field):
                    field = field.unwrap()
                if isinstance(field, Field):
                    accum.append(field.column_name)

        if not accum:
            raise ValueError('Unable to generate a name for the index, please '
                             'explicitly specify a name.')

        index_name = re.sub('[^\w]+', '',
                            '%s_%s' % (model._meta.name, '_'.join(accum)))
        if len(index_name) > 64:
            index_hash = hashlib.md5(index_name.encode('utf-8')).hexdigest()
            index_name = '%s_%s' % (index_name[:56], index_hash[:7])
        return index_name


# DB-API 2.0 EXCEPTIONS.


class PeeweeException(Exception): pass
class ImproperlyConfigured(PeeweeException): pass
class DatabaseError(PeeweeException): pass
class DataError(DatabaseError): pass
class IntegrityError(DatabaseError): pass
class InterfaceError(PeeweeException): pass
class InternalError(DatabaseError): pass
class NotSupportedError(DatabaseError): pass
class OperationalError(DatabaseError): pass
class ProgrammingError(DatabaseError): pass


class ExceptionWrapper(object):
    __slots__ = ('exceptions',)
    def __init__(self, exceptions):
        self.exceptions = exceptions
    def __enter__(self): pass
    def __exit__(self, exc_type, exc_value, traceback):
        if exc_type is None:
            return
        if exc_type.__name__ in self.exceptions:
            new_type = self.exceptions[exc_type.__name__]
            exc_args = exc_value.args
            reraise(new_type, new_type(*exc_args), traceback)

EXCEPTIONS = {
    'ConstraintError': IntegrityError,
    'DatabaseError': DatabaseError,
    'DataError': DataError,
    'IntegrityError': IntegrityError,
    'InterfaceError': InterfaceError,
    'InternalError': InternalError,
    'NotSupportedError': NotSupportedError,
    'OperationalError': OperationalError,
    'ProgrammingError': ProgrammingError}

__exception_wrapper__ = ExceptionWrapper(EXCEPTIONS)


# DATABASE INTERFACE AND CONNECTION MANAGEMENT.


IndexMetadata = collections.namedtuple(
    'IndexMetadata',
    ('name', 'sql', 'columns', 'unique', 'table'))
ColumnMetadata = collections.namedtuple(
    'ColumnMetadata',
    ('name', 'data_type', 'null', 'primary_key', 'table'))
ForeignKeyMetadata = collections.namedtuple(
    'ForeignKeyMetadata',
    ('column', 'dest_table', 'dest_column', 'table'))


class _ConnectionState(object):
    def __init__(self, **kwargs):
        super(_ConnectionState, self).__init__(**kwargs)
        self.reset()

    def reset(self):
        self.closed = True
        self.conn = None
        self.transactions = []

    def set_connection(self, conn):
        self.conn = conn
        self.closed = False


class _ConnectionLocal(_ConnectionState, threading.local): pass
class _NoopLock(object):
    __slots__ = ()
    def __enter__(self): return self
    def __exit__(self, exc_type, exc_val, exc_tb): pass


class ConnectionContext(_callable_context_manager):
    __slots__ = ('db',)
    def __init__(self, db): self.db = db
    def __enter__(self):
        if self.db.is_closed():
            self.db.connect()
    def __exit__(self, exc_type, exc_val, exc_tb): self.db.close()


class Database(_callable_context_manager):
    context_class = Context
    field_types = {}
    operations = {}
    param = '?'
    quote = '"'

    # Feature toggles.
    commit_select = False
    compound_select_parentheses = False
    for_update = False
    limit_max = None
    returning_clause = False
    safe_create_index = True
    safe_drop_index = True
    sequences = False

    def __init__(self, database, thread_safe=True, autorollback=False,
                 field_types=None, operations=None, autocommit=None, **kwargs):
        self._field_types = merge_dict(FIELD, self.field_types)
        self._operations = merge_dict(OP, self.operations)
        if field_types:
            self._field_types.update(field_types)
        if operations:
            self._operations.update(operations)

        self.autorollback = autorollback
        self.thread_safe = thread_safe
        if thread_safe:
            self._state = _ConnectionLocal()
            self._lock = threading.Lock()
        else:
            self._state = _ConnectionState()
            self._lock = _NoopLock()

        if autocommit is not None:
            __deprecated__('Peewee no longer uses the "autocommit" option, as '
                           'the semantics now require it to always be True. '
                           'Because some database-drivers also use the '
                           '"autocommit" parameter, you are receiving a '
                           'warning so you may update your code and remove '
                           'the parameter, as in the future, specifying '
                           'autocommit could impact the behavior of the '
                           'database driver you are using.')

        self.connect_params = {}
        self.init(database, **kwargs)

    def init(self, database, **kwargs):
        if not self.is_closed():
            self.close()
        self.database = database
        self.connect_params.update(kwargs)
        self.deferred = not bool(database)

    def __enter__(self):
        if self.is_closed():
            self.connect()
        self.transaction().__enter__()
        return self

    def __exit__(self, exc_type, exc_val, exc_tb):
        top = self._state.transactions[-1]
        try:
            top.__exit__(exc_type, exc_val, exc_tb)
        finally:
            self.close()

    def connection_context(self):
        return ConnectionContext(self)

    def _connect(self):
        raise NotImplementedError

    def connect(self, reuse_if_open=False):
        with self._lock:
            if self.deferred:
                raise Exception('Error, database must be initialized before '
                                'opening a connection.')
            if not self._state.closed:
                if reuse_if_open:
                    return False
                raise OperationalError('Connection already opened.')

            self._state.reset()
            with __exception_wrapper__:
                self._state.set_connection(self._connect())
                self._initialize_connection(self._state.conn)
        return True

    def _initialize_connection(self, conn):
        pass

    def close(self):
        with self._lock:
            if self.deferred:
                raise Exception('Error, database must be initialized before '
                                'opening a connection.')
            if self.in_transaction():
                raise OperationalError('Attempting to close database while '
                                       'transaction is open.')
            is_open = not self._state.closed
            try:
                if is_open:
                    with __exception_wrapper__:
                        self._close(self._state.conn)
            finally:
                self._state.reset()
            return is_open

    def _close(self, conn):
        conn.close()

    def is_closed(self):
        return self._state.closed

    def connection(self):
        if self.is_closed():
            self.connect()
        return self._state.conn

    def cursor(self, commit=None):
        if self.is_closed():
            self.connect()
        return self._state.conn.cursor()

    def execute_sql(self, sql, params=None, commit=SENTINEL):
        logger.debug((sql, params))
        if commit is SENTINEL:
            if self.in_transaction():
                commit = False
            elif self.commit_select:
                commit = True
            else:
                commit = not sql[:6].lower().startswith('select')

        with __exception_wrapper__:
            cursor = self.cursor(commit)
            try:
                cursor.execute(sql, params or ())
            except Exception:
                if self.autorollback and not self.in_transaction():
                    self.rollback()
                raise
            else:
                if commit and not self.in_transaction():
                    self.commit()
        return cursor

    def execute(self, query, commit=SENTINEL, **context_options):
        ctx = self.get_sql_context(**context_options)
        sql, params = ctx.sql(query).query()
        return self.execute_sql(sql, params, commit=commit)

    def get_context_options(self):
        return {
            'field_types': self._field_types,
            'operations': self._operations,
            'param': self.param,
            'quote': self.quote,
            'compound_select_parentheses': self.compound_select_parentheses,
            'conflict_statement': self.conflict_statement,
            'conflict_update': self.conflict_update,
            'for_update': self.for_update,
            'limit_max': self.limit_max,
        }

    def get_sql_context(self, **context_options):
        context = self.get_context_options()
        if context_options:
            context.update(context_options)
        return self.context_class(**context)

    def conflict_statement(self, on_conflict):
        raise NotImplementedError

    def conflict_update(self, on_conflict):
        raise NotImplementedError

    def last_insert_id(self, cursor, query_type=None):
        return cursor.lastrowid

    def rows_affected(self, cursor):
        return cursor.rowcount

    def default_values_insert(self, ctx):
        return ctx.literal('DEFAULT VALUES')

    def in_transaction(self):
        return bool(self._state.transactions)

    def push_transaction(self, transaction):
        self._state.transactions.append(transaction)

    def pop_transaction(self):
        return self._state.transactions.pop()

    def transaction_depth(self):
        return len(self._state.transactions)

    def top_transaction(self):
        if self._state.transactions:
            return self._state.transactions[-1]

    def atomic(self):
        return _atomic(self)

    def manual_commit(self):
        return _manual(self)

    def transaction(self):
        return _transaction(self)

    def savepoint(self):
        return _savepoint(self)

    def begin(self):
        if self.is_closed():
            self.connect()

    def commit(self):
        return self._state.conn.commit()

    def rollback(self):
        return self._state.conn.rollback()

    def batch_commit(self, it, n):
        for group in chunked(it, n):
            with self.atomic():
                for obj in group:
                    yield obj

    def table_exists(self, table, schema=None):
        return table.__name__ in self.get_tables(schema=schema)

    def get_tables(self, schema=None):
        raise NotImplementedError

    def get_indexes(self, table, schema=None):
        raise NotImplementedError

    def get_columns(self, table, schema=None):
        raise NotImplementedError

    def get_primary_keys(self, table, schema=None):
        raise NotImplementedError

    def get_foreign_keys(self, table, schema=None):
        raise NotImplementedError

    def sequence_exists(self, seq):
        raise NotImplementedError

    def create_tables(self, models, **options):
        for model in sort_models(models):
            model.create_table(**options)

    def drop_tables(self, models, **kwargs):
        for model in reversed(sort_models(models)):
            model.drop_table(**kwargs)

    def extract_date(self, date_part, date_field):
        raise NotImplementedError

    def truncate_date(self, date_part, date_field):
        raise NotImplementedError

    def bind(self, models, bind_refs=True, bind_backrefs=True):
        for model in models:
            model.bind(self, bind_refs=bind_refs, bind_backrefs=bind_backrefs)

    def bind_ctx(self, models, bind_refs=True, bind_backrefs=True):
        return _BoundModelsContext(models, self, bind_refs, bind_backrefs)

    def get_noop_select(self, ctx):
        return ctx.sql(Select().columns(SQL('0')).where(SQL('0')))


def __pragma__(name):
    def __get__(self):
        return self.pragma(name)
    def __set__(self, value):
        return self.pragma(name, value)
    return property(__get__, __set__)


class SqliteDatabase(Database):
    field_types = {
        'BIGAUTO': FIELD.AUTO,
        'BIGINT': FIELD.INT,
        'BOOL': FIELD.INT,
        'DOUBLE': FIELD.FLOAT,
        'SMALLINT': FIELD.INT,
        'UUID': FIELD.TEXT}
    operations = {
        'LIKE': 'GLOB',
        'ILIKE': 'LIKE'}
    limit_max = -1

    def __init__(self, database, *args, **kwargs):
        self._pragmas = kwargs.pop('pragmas', ())
        super(SqliteDatabase, self).__init__(database, *args, **kwargs)
        self._aggregates = {}
        self._collations = {}
        self._functions = {}
        self._table_functions = []
        self._extensions = set()
        self.register_function(_sqlite_date_part, 'date_part', 2)
        self.register_function(_sqlite_date_trunc, 'date_trunc', 2)

    def init(self, database, pragmas=None, timeout=5, **kwargs):
        if pragmas is not None:
            self._pragmas = pragmas
        self._timeout = timeout
        super(SqliteDatabase, self).init(database, **kwargs)

    def _connect(self):
        if sqlite3 is None:
            raise ImproperlyConfigured('SQLite driver not installed!')
        conn = sqlite3.connect(self.database, timeout=self._timeout,
                               **self.connect_params)
        conn.isolation_level = None
        try:
            self._add_conn_hooks(conn)
        except:
            conn.close()
            raise
        return conn

    def _add_conn_hooks(self, conn):
        self._set_pragmas(conn)
        self._load_aggregates(conn)
        self._load_collations(conn)
        self._load_functions(conn)
        if self._table_functions:
            for table_function in self._table_functions:
                table_function.register(conn)
        if self._extensions:
            self._load_extensions(conn)

    def _set_pragmas(self, conn):
        if self._pragmas:
            cursor = conn.cursor()
            for pragma, value in self._pragmas:
                cursor.execute('PRAGMA %s = %s;' % (pragma, value))
            cursor.close()

    def pragma(self, key, value=SENTINEL, permanent=False):
        sql = 'PRAGMA %s' % key
        if value is not SENTINEL:
            sql += ' = %s' % (value or 0)
            if permanent:
                pragmas = dict(self._pragmas or ())
                pragmas[key] = value
                self._pragmas = list(pragmas.items())
        elif permanent:
            raise ValueError('Cannot specify a permanent pragma without value')
        row = self.execute_sql(sql).fetchone()
        if row:
            return row[0]

    cache_size = __pragma__('cache_size')
    foreign_keys = __pragma__('foreign_keys')
    journal_mode = __pragma__('journal_mode')
    journal_size_limit = __pragma__('journal_size_limit')
    mmap_size = __pragma__('mmap_size')
    page_size = __pragma__('page_size')
    read_uncommitted = __pragma__('read_uncommitted')
    synchronous = __pragma__('synchronous')
    wal_autocheckpoint = __pragma__('wal_autocheckpoint')

    @property
    def timeout(self):
        return self._timeout

    @timeout.setter
    def timeout(self, seconds):
        if self._timeout == seconds:
            return

        self._timeout = seconds
        if not self.is_closed():
            self.execute_sql('PRAGMA busy_timeout=%d;' % (seconds * 1000))

    def _load_aggregates(self, conn):
        for name, (klass, num_params) in self._aggregates.items():
            conn.create_aggregate(name, num_params, klass)

    def _load_collations(self, conn):
        for name, fn in self._collations.items():
            conn.create_collation(name, fn)

    def _load_functions(self, conn):
        for name, (fn, num_params) in self._functions.items():
            conn.create_function(name, num_params, fn)

    def register_aggregate(self, klass, name=None, num_params=-1):
        self._aggregates[name or klass.__name__.lower()] = (klass, num_params)
        if not self.is_closed():
            self._load_aggregates(self.connection())

    def aggregate(self, name=None, num_params=-1):
        def decorator(klass):
            self.register_aggregate(klass, name, num_params)
            return klass
        return decorator

    def register_collation(self, fn, name=None):
        name = name or fn.__name__
        def _collation(*args):
            expressions = args + (SQL('collate %s' % name),)
            return NodeList(expressions)
        fn.collation = _collation
        self._collations[name] = fn
        if not self.is_closed():
            self._load_collations(self.connection())

    def collation(self, name=None):
        def decorator(fn):
            self.register_collation(fn, name)
            return fn
        return decorator

    def register_function(self, fn, name=None, num_params=-1):
        self._functions[name or fn.__name__] = (fn, num_params)
        if not self.is_closed():
            self._load_functions(self.connection())

    def func(self, name=None, num_params=-1):
        def decorator(fn):
            self.register_function(fn, name, num_params)
            return fn
        return decorator

    def register_table_function(self, klass, name=None):
        if name is not None:
            klass.name = name
        self._table_functions.append(klass)
        if not self.is_closed():
            klass.register(self.connection())

    def table_function(self, name=None):
        def decorator(klass):
            self.register_table_function(klass, name)
            return klass
        return decorator

    def unregister_aggregate(self, name):
        del(self._aggregates[name])

    def unregister_collation(self, name):
        del(self._collations[name])

    def unregister_function(self, name):
        del(self._functions[name])

    def unregister_table_function(self, name):
        for idx, klass in enumerate(self._table_functions):
            if klass.name == name:
                break
        else:
            return False
        self._table_functions.pop(idx)
        return True

    def _load_extensions(self, conn):
        conn.enable_load_extension(True)
        for extension in self._extensions:
            conn.load_extension(extension)

    def load_extension(self, extension):
        self._extensions.add(extension)
        if not self.is_closed():
            conn = self.connection()
            conn.enable_load_extension(True)
            conn.load_extension(extension)

    def unload_extension(self, extension):
        self._extensions.remove(extension)

    def atomic(self, lock_type=None):
        return _atomic(self, lock_type=lock_type)

    def transaction(self, lock_type=None):
        return _transaction(self, lock_type=lock_type)

    def begin(self, lock_type=None):
        statement = 'BEGIN %s' % lock_type if lock_type else 'BEGIN'
        self.execute_sql(statement, commit=False)

    def get_tables(self, schema=None):
        cursor = self.execute_sql('SELECT name FROM sqlite_master WHERE '
                                  'type = ? ORDER BY name;', ('table',))
        return [row for row, in cursor.fetchall()]

    def get_indexes(self, table, schema=None):
        query = ('SELECT name, sql FROM sqlite_master '
                 'WHERE tbl_name = ? AND type = ? ORDER BY name')
        cursor = self.execute_sql(query, (table, 'index'))
        index_to_sql = dict(cursor.fetchall())

        # Determine which indexes have a unique constraint.
        unique_indexes = set()
        cursor = self.execute_sql('PRAGMA index_list("%s")' % table)
        for row in cursor.fetchall():
            name = row[1]
            is_unique = int(row[2]) == 1
            if is_unique:
                unique_indexes.add(name)

        # Retrieve the indexed columns.
        index_columns = {}
        for index_name in sorted(index_to_sql):
            cursor = self.execute_sql('PRAGMA index_info("%s")' % index_name)
            index_columns[index_name] = [row[2] for row in cursor.fetchall()]

        return [
            IndexMetadata(
                name,
                index_to_sql[name],
                index_columns[name],
                name in unique_indexes,
                table)
            for name in sorted(index_to_sql)]

    def get_columns(self, table, schema=None):
        cursor = self.execute_sql('PRAGMA table_info("%s")' % table)
        return [ColumnMetadata(row[1], row[2], not row[3], bool(row[5]), table)
                for row in cursor.fetchall()]

    def get_primary_keys(self, table, schema=None):
        cursor = self.execute_sql('PRAGMA table_info("%s")' % table)
        return [row[1] for row in filter(lambda r: r[-1], cursor.fetchall())]

    def get_foreign_keys(self, table, schema=None):
        cursor = self.execute_sql('PRAGMA foreign_key_list("%s")' % table)
        return [ForeignKeyMetadata(row[3], row[2], row[4], table)
                for row in cursor.fetchall()]

    def get_binary_type(self):
        return sqlite3.Binary

    def conflict_statement(self, on_conflict):
        if on_conflict._action:
            return SQL('INSERT OR %s' % on_conflict._action.upper())

    def conflict_update(self, on_conflict):
        if any((on_conflict._preserve, on_conflict._update, on_conflict._where,
                on_conflict._conflict_target)):
            raise ValueError('SQLite does not support specifying which values '
                             'to preserve or update.')

    def extract_date(self, date_part, date_field):
        return fn.date_part(date_part, date_field)

    def truncate_date(self, date_part, date_field):
        return fn.date_trunc(date_part, date_field)


class PostgresqlDatabase(Database):
    field_types = {
        'AUTO': 'SERIAL',
        'BIGAUTO': 'BIGSERIAL',
        'BLOB': 'BYTEA',
        'BOOL': 'BOOLEAN',
        'DATETIME': 'TIMESTAMP',
        'DECIMAL': 'NUMERIC',
        'DOUBLE': 'DOUBLE PRECISION',
        'UUID': 'UUID'}
    operations = {'REGEXP': '~'}
    param = '%s'

    commit_select = True
    compound_select_parentheses = True
    for_update = True
    returning_clause = True
    safe_create_index = False
    sequences = True

    def init(self, database, register_unicode=True, encoding=None, **kwargs):
        self._register_unicode = register_unicode
        self._encoding = encoding
        self._need_server_version = True
        super(PostgresqlDatabase, self).init(database, **kwargs)

    def _connect(self):
        if psycopg2 is None:
            raise ImproperlyConfigured('Postgres driver not installed!')
        conn = psycopg2.connect(database=self.database, **self.connect_params)
        if self._register_unicode:
            pg_extensions.register_type(pg_extensions.UNICODE, conn)
            pg_extensions.register_type(pg_extensions.UNICODEARRAY, conn)
        if self._encoding:
            conn.set_client_encoding(self._encoding)
        if self._need_server_version:
            self.set_server_version(conn.server_version)
            self._need_server_version = False
        return conn

    def set_server_version(self, version):
        if version >= 90600:
            self.safe_create_index = True

    def last_insert_id(self, cursor, query_type=None):
        try:
            return cursor if query_type else cursor[0][0]
        except (IndexError, KeyError, TypeError):
            pass

    def get_tables(self, schema=None):
        query = ('SELECT tablename FROM pg_catalog.pg_tables '
                 'WHERE schemaname = %s ORDER BY tablename')
        cursor = self.execute_sql(query, (schema or 'public',))
        return [table for table, in cursor.fetchall()]

    def get_indexes(self, table, schema=None):
        query = """
            SELECT
                i.relname, idxs.indexdef, idx.indisunique,
                array_to_string(array_agg(cols.attname), ',')
            FROM pg_catalog.pg_class AS t
            INNER JOIN pg_catalog.pg_index AS idx ON t.oid = idx.indrelid
            INNER JOIN pg_catalog.pg_class AS i ON idx.indexrelid = i.oid
            INNER JOIN pg_catalog.pg_indexes AS idxs ON
                (idxs.tablename = t.relname AND idxs.indexname = i.relname)
            LEFT OUTER JOIN pg_catalog.pg_attribute AS cols ON
                (cols.attrelid = t.oid AND cols.attnum = ANY(idx.indkey))
            WHERE t.relname = %s AND t.relkind = %s AND idxs.schemaname = %s
            GROUP BY i.relname, idxs.indexdef, idx.indisunique
            ORDER BY idx.indisunique DESC, i.relname;"""
        cursor = self.execute_sql(query, (table, 'r', schema or 'public'))
        return [IndexMetadata(row[0], row[1], row[3].split(','), row[2], table)
                for row in cursor.fetchall()]

    def get_columns(self, table, schema=None):
        query = """
            SELECT column_name, is_nullable, data_type
            FROM information_schema.columns
            WHERE table_name = %s AND table_schema = %s
            ORDER BY ordinal_position"""
        cursor = self.execute_sql(query, (table, schema or 'public'))
        pks = set(self.get_primary_keys(table, schema))
        return [ColumnMetadata(name, dt, null == 'YES', name in pks, table)
                for name, null, dt in cursor.fetchall()]

    def get_primary_keys(self, table, schema=None):
        query = """
            SELECT kc.column_name
            FROM information_schema.table_constraints AS tc
            INNER JOIN information_schema.key_column_usage AS kc ON (
                tc.table_name = kc.table_name AND
                tc.table_schema = kc.table_schema AND
                tc.constraint_name = kc.constraint_name)
            WHERE
                tc.constraint_type = %s AND
                tc.table_name = %s AND
                tc.table_schema = %s"""
        ctype = 'PRIMARY KEY'
        cursor = self.execute_sql(query, (ctype, table, schema or 'public'))
        return [pk for pk, in cursor.fetchall()]

    def get_foreign_keys(self, table, schema=None):
        sql = """
            SELECT
                kcu.column_name, ccu.table_name, ccu.column_name
            FROM information_schema.table_constraints AS tc
            JOIN information_schema.key_column_usage AS kcu
                ON (tc.constraint_name = kcu.constraint_name AND
                    tc.constraint_schema = kcu.constraint_schema)
            JOIN information_schema.constraint_column_usage AS ccu
                ON (ccu.constraint_name = tc.constraint_name AND
                    ccu.constraint_schema = tc.constraint_schema)
            WHERE
                tc.constraint_type = 'FOREIGN KEY' AND
                tc.table_name = %s AND
                tc.table_schema = %s"""
        cursor = self.execute_sql(sql, (table, schema or 'public'))
        return [ForeignKeyMetadata(row[0], row[1], row[2], table)
                for row in cursor.fetchall()]

    def sequence_exists(self, sequence):
        res = self.execute_sql("""
            SELECT COUNT(*) FROM pg_class, pg_namespace
            WHERE relkind='S'
                AND pg_class.relnamespace = pg_namespace.oid
                AND relname=%s""", (sequence,))
        return bool(res.fetchone()[0])

    def get_binary_type(self):
        return psycopg2.Binary

    def conflict_statement(self, on_conflict):
        return

    def conflict_update(self, on_conflict):
        action = on_conflict._action.lower() if on_conflict._action else ''
        if action in ('ignore', 'nothing'):
            return SQL('ON CONFLICT DO NOTHING')
        elif action and action != 'update':
            raise ValueError('The only supported actions for conflict '
                             'resolution with Postgresql are "ignore" or '
                             '"update".')
        elif not on_conflict._update and not on_conflict._preserve:
            raise ValueError('If you are not performing any updates (or '
                             'preserving any INSERTed values), then the '
                             'conflict resolution action should be set to '
                             '"IGNORE".')
        elif not on_conflict._conflict_target:
            raise ValueError('Postgres requires that a conflict target be '
                             'specified when doing an upsert.')

        target = EnclosedNodeList([
            Entity(col) if isinstance(col, basestring) else col
            for col in on_conflict._conflict_target])

        updates = []
        if on_conflict._preserve:
            for column in on_conflict._preserve:
                excluded = NodeList((SQL('EXCLUDED'), ensure_entity(column)),
                                    glue='.')
                expression = NodeList((ensure_entity(column), SQL('='),
                                       excluded))
                updates.append(expression)

        if on_conflict._update:
            for k, v in on_conflict._update.items():
                if not isinstance(v, Node):
                    converter = k.db_value if isinstance(k, Field) else None
                    v = Value(v, converter=converter, unpack=False)
                else:
                    v = QualifiedNames(v)
                updates.append(NodeList((ensure_entity(k), SQL('='), v)))

        parts = [SQL('ON CONFLICT'),
                 target,
                 SQL('DO UPDATE SET'),
                 CommaNodeList(updates)]
        if on_conflict._where:
            parts.extend((SQL('WHERE'), QualifiedNames(on_conflict._where)))

        return NodeList(parts)

    def extract_date(self, date_part, date_field):
        return fn.EXTRACT(NodeList((date_part, SQL('FROM'), date_field)))

    def truncate_date(self, date_part, date_field):
        return fn.DATE_TRUNC(date_part, date_field)

    def get_noop_select(self, ctx):
        return ctx.sql(Select().columns(SQL('0')).where(SQL('false')))


class MySQLDatabase(Database):
    field_types = {
        'AUTO': 'INTEGER AUTO_INCREMENT',
        'BIGAUTO': 'BIGINT AUTO_INCREMENT',
        'BOOL': 'BOOL',
        'DECIMAL': 'NUMERIC',
        'DOUBLE': 'DOUBLE PRECISION',
        'FLOAT': 'FLOAT',
        'UUID': 'VARCHAR(40)'}
    operations = {
        'LIKE': 'LIKE BINARY',
        'ILIKE': 'LIKE',
        'XOR': 'XOR'}
    param = '%s'
    quote = '`'

    commit_select = True
    for_update = True
    limit_max = 2 ** 64 - 1
    safe_create_index = False
    safe_drop_index = False

    def init(self, database, **kwargs):
        params = {'charset': 'utf8', 'use_unicode': True}
        params.update(kwargs)
        if 'password' in params:
            params['passwd'] = params.pop('password')
        super(MySQLDatabase, self).init(database, **params)

    def _connect(self):
        if mysql is None:
            raise ImproperlyConfigured('MySQL driver not installed!')
        return mysql.connect(db=self.database, **self.connect_params)

    def default_values_insert(self, ctx):
        return ctx.literal('() VALUES ()')

    def get_tables(self, schema=None):
        return [table for table, in self.execute_sql('SHOW TABLES')]

    def get_indexes(self, table, schema=None):
        cursor = self.execute_sql('SHOW INDEX FROM `%s`' % table)
        unique = set()
        indexes = {}
        for row in cursor.fetchall():
            if not row[1]:
                unique.add(row[2])
            indexes.setdefault(row[2], [])
            indexes[row[2]].append(row[4])
        return [IndexMetadata(name, None, indexes[name], name in unique, table)
                for name in indexes]

    def get_columns(self, table, schema=None):
        sql = """
            SELECT column_name, is_nullable, data_type
            FROM information_schema.columns
            WHERE table_name = %s AND table_schema = DATABASE()"""
        cursor = self.execute_sql(sql, (table,))
        pks = set(self.get_primary_keys(table))
        return [ColumnMetadata(name, dt, null == 'YES', name in pks, table)
                for name, null, dt in cursor.fetchall()]

    def get_primary_keys(self, table, schema=None):
        cursor = self.execute_sql('SHOW INDEX FROM `%s`' % table)
        return [row[4] for row in
                filter(lambda row: row[2] == 'PRIMARY', cursor.fetchall())]

    def get_foreign_keys(self, table, schema=None):
        query = """
            SELECT column_name, referenced_table_name, referenced_column_name
            FROM information_schema.key_column_usage
            WHERE table_name = %s
                AND table_schema = DATABASE()
                AND referenced_table_name IS NOT NULL
                AND referenced_column_name IS NOT NULL"""
        cursor = self.execute_sql(query, (table,))
        return [
            ForeignKeyMetadata(column, dest_table, dest_column, table)
            for column, dest_table, dest_column in cursor.fetchall()]

    def get_binary_type(self):
        return mysql.Binary

    def conflict_statement(self, on_conflict):
        if not on_conflict._action: return

        action = on_conflict._action.lower()
        if action == 'replace':
            return SQL('REPLACE')
        elif action == 'ignore':
            return SQL('INSERT IGNORE')
        elif action != 'update':
            raise ValueError('Un-supported action for conflict resolution. '
                             'MySQL supports REPLACE, IGNORE and UPDATE.')

    def conflict_update(self, on_conflict):
        if on_conflict._where or on_conflict._conflict_target:
            raise ValueError('MySQL does not support the specification of '
                             'where clauses or conflict targets for conflict '
                             'resolution.')

        updates = []
        if on_conflict._preserve:
            for column in on_conflict._preserve:
                entity = ensure_entity(column)
                expression = NodeList((
                    ensure_entity(column),
                    SQL('='),
                    fn.VALUES(entity)))
                updates.append(expression)

        if on_conflict._update:
            for k, v in on_conflict._update.items():
                if not isinstance(v, Node):
                    converter = k.db_value if isinstance(k, Field) else None
                    v = Value(v, converter=converter, unpack=False)
                updates.append(NodeList((ensure_entity(k), SQL('='), v)))

        if updates:
            return NodeList((SQL('ON DUPLICATE KEY UPDATE'),
                             CommaNodeList(updates)))

    def extract_date(self, date_part, date_field):
        return fn.EXTRACT(NodeList((SQL(date_part), SQL('FROM'), date_field)))

    def truncate_date(self, date_part, date_field):
        return fn.DATE_FORMAT(date_field, __mysql_date_trunc__[date_part])

    def get_noop_select(self, ctx):
        return ctx.literal('DO 0')


# TRANSACTION CONTROL.


class _manual(_callable_context_manager):
    def __init__(self, db):
        self.db = db

    def __enter__(self):
        top = self.db.top_transaction()
        if top and not isinstance(self.db.top_transaction(), _manual):
            raise ValueError('Cannot enter manual commit block while a '
                             'transaction is active.')
        self.db.push_transaction(self)

    def __exit__(self, exc_type, exc_val, exc_tb):
        if self.db.pop_transaction() is not self:
            raise ValueError('Transaction stack corrupted while exiting '
                             'manual commit block.')


class _atomic(_callable_context_manager):
    def __init__(self, db, lock_type=None):
        self.db = db
        self._lock_type = lock_type
        self._transaction_args = (lock_type,) if lock_type is not None else ()

    def __enter__(self):
        if self.db.transaction_depth() == 0:
            self._helper = self.db.transaction(*self._transaction_args)
        else:
            self._helper = self.db.savepoint()
            if isinstance(self.db.top_transaction(), _manual):
                raise ValueError('Cannot enter atomic commit block while in '
                                 'manual commit mode.')
        return self._helper.__enter__()

    def __exit__(self, exc_type, exc_val, exc_tb):
        return self._helper.__exit__(exc_type, exc_val, exc_tb)


class _transaction(_callable_context_manager):
    def __init__(self, db, lock_type=None):
        self.db = db
        self._lock_type = lock_type

    def _begin(self):
        if self._lock_type:
            self.db.begin(self._lock_type)
        else:
            self.db.begin()

    def commit(self, begin=True):
        self.db.commit()
        if begin:
            self._begin()

    def rollback(self, begin=True):
        self.db.rollback()
        if begin:
            self._begin()

    def __enter__(self):
        if self.db.transaction_depth() == 0:
            self._begin()
        self.db.push_transaction(self)
        return self

    def __exit__(self, exc_type, exc_val, exc_tb):
        try:
            if exc_type:
                self.rollback(False)
            elif self.db.transaction_depth() == 1:
                try:
                    self.commit(False)
                except:
                    self.rollback(False)
                    raise
        finally:
            self.db.pop_transaction()


class _savepoint(_callable_context_manager):
    def __init__(self, db, sid=None):
        self.db = db
        self.sid = sid or 's' + uuid.uuid4().hex
        self.quoted_sid = self.sid.join((self.db.quote, self.db.quote))

    def _begin(self):
        self.db.execute_sql('SAVEPOINT %s;' % self.quoted_sid)

    def commit(self, begin=True):
        self.db.execute_sql('RELEASE SAVEPOINT %s;' % self.quoted_sid)
        if begin: self._begin()

    def rollback(self):
        self.db.execute_sql('ROLLBACK TO SAVEPOINT %s;' % self.quoted_sid)

    def __enter__(self):
        self._begin()
        return self

    def __exit__(self, exc_type, exc_val, exc_tb):
        if exc_type:
            self.rollback()
        else:
            try:
                self.commit(begin=False)
            except:
                self.rollback()
                raise


# CURSOR REPRESENTATIONS.


class CursorWrapper(object):
    def __init__(self, cursor):
        self.cursor = cursor
        self.count = 0
        self.index = 0
        self.initialized = False
        self.populated = False
        self.row_cache = []

    def __iter__(self):
        if self.populated:
            return iter(self.row_cache)
        return ResultIterator(self)

    def __getitem__(self, item):
        if isinstance(item, slice):
            # TODO: getslice
            start = item.start
            stop = item.stop
            if stop is None or stop < 0:
                self.fill_cache()
            else:
                self.fill_cache(stop)
            return self.row_cache[item]
        elif isinstance(item, int):
            self.fill_cache(item if item > 0 else 0)
            return self.row_cache[item]
        else:
            raise ValueError('CursorWrapper only supports integer and slice '
                             'indexes.')

    def __len__(self):
        self.fill_cache()
        return self.count

    def initialize(self):
        pass

    def iterate(self, cache=True):
        row = self.cursor.fetchone()
        if row is None:
            self.populated = True
            self.cursor.close()
            raise StopIteration
        elif not self.initialized:
            self.initialize()  # Lazy initialization.
            self.initialized = True
        self.count += 1
        result = self.process_row(row)
        if cache:
            self.row_cache.append(result)
        return result

    def process_row(self, row):
        return row

    def iterator(self):
        """Efficient one-pass iteration over the result set."""
        while True:
            yield self.iterate(False)

    def fill_cache(self, n=0.):
        n = n or float('Inf')
        if n < 0:
            raise ValueError('Negative values are not supported.')

        iterator = ResultIterator(self)
        iterator.index = self.count
        while not self.populated and (n > self.count):
            try:
                iterator.next()
            except StopIteration:
                break


class DictCursorWrapper(CursorWrapper):
    def _initialize_columns(self):
        description = self.cursor.description
        self.columns = [t[0][t[0].find('.') + 1:]
                        for t in description]
        self.ncols = len(description)

    initialize = _initialize_columns

    def _row_to_dict(self, row):
        result = {}
        for i in range(self.ncols):
            result[self.columns[i]] = row[i]
        return result

    process_row = _row_to_dict


class NamedTupleCursorWrapper(CursorWrapper):
    def initialize(self):
        description = self.cursor.description
        self.tuple_class = collections.namedtuple(
            'Row',
            [col[0][col[0].find('.') + 1:].strip('"') for col in description])

    def process_row(self, row):
        return self.tuple_class(*row)


class ObjectCursorWrapper(DictCursorWrapper):
    def __init__(self, cursor, constructor):
        super(ObjectCursorWrapper, self).__init__(cursor)
        self.constructor = constructor

    def process_row(self, row):
        row_dict = self._row_to_dict(row)
        return self.constructor(**row_dict)


class ResultIterator(object):
    def __init__(self, cursor_wrapper):
        self.cursor_wrapper = cursor_wrapper
        self.index = 0

    def __iter__(self):
        return self

    def next(self):
        if self.index < self.cursor_wrapper.count:
            obj = self.cursor_wrapper.row_cache[self.index]
        elif not self.cursor_wrapper.populated:
            self.cursor_wrapper.iterate()
            obj = self.cursor_wrapper.row_cache[self.index]
        else:
            raise StopIteration
        self.index += 1
        return obj

    __next__ = next


# FIELDS


class FieldAccessor(object):
    def __init__(self, model, field, name):
        self.model = model
        self.field = field
        self.name = name

    def __get__(self, instance, instance_type=None):
        if instance is not None:
            return instance.__data__.get(self.name)
        return self.field

    def __set__(self, instance, value):
        instance.__data__[self.name] = value
        instance._dirty.add(self.name)


class ForeignKeyAccessor(FieldAccessor):
    def __init__(self, model, field, name):
        super(ForeignKeyAccessor, self).__init__(model, field, name)
        self.rel_model = field.rel_model

    def get_rel_instance(self, instance):
        value = instance.__data__.get(self.name)
        if value is not None or self.name in instance.__rel__:
            if self.name not in instance.__rel__:
                obj = self.rel_model.get(self.field.rel_field == value)
                instance.__rel__[self.name] = obj
            return instance.__rel__[self.name]
        elif not self.field.null:
            raise self.rel_model.DoesNotExist
        return value

    def __get__(self, instance, instance_type=None):
        if instance is not None:
            return self.get_rel_instance(instance)
        return self.field

    def __set__(self, instance, obj):
        if isinstance(obj, self.rel_model):
            instance.__data__[self.name] = getattr(obj, self.field.rel_field.name)
            instance.__rel__[self.name] = obj
        else:
            fk_value = instance.__data__.get(self.name)
            instance.__data__[self.name] = obj
            if obj != fk_value and self.name in instance.__rel__:
                del instance.__rel__[self.name]
        instance._dirty.add(self.name)


class BackrefAccessor(object):
    def __init__(self, field):
        self.field = field
        self.model = field.rel_model
        self.rel_model = field.model

    def __get__(self, instance, instance_type=None):
        if instance is not None:
            dest = self.field.rel_field.name
            return (self.rel_model
                    .select()
                    .where(self.field == getattr(instance, dest)))
        return self


class ObjectIdAccessor(object):
    """Gives direct access to the underlying id"""
    def __init__(self, field):
        self.field = field

    def __get__(self, instance, instance_type=None):
        if instance is not None:
            return instance.__data__.get(self.field.name)
        return self.field

    def __set__(self, instance, value):
        setattr(instance, self.field.name, value)


class Field(ColumnBase):
    _field_counter = 0
    _order = 0
    accessor_class = FieldAccessor
    auto_increment = False
    field_type = 'DEFAULT'

    def __init__(self, null=False, index=False, unique=False, column_name=None,
                 default=None, primary_key=False, constraints=None,
                 sequence=None, collation=None, unindexed=False, choices=None,
                 help_text=None, verbose_name=None, db_column=None,
                 _hidden=False):
        if db_column is not None:
            __deprecated__('"db_column" has been deprecated in favor of '
                           '"column_name" for Field objects.')
            column_name = db_column

        self.null = null
        self.index = index
        self.unique = unique
        self.column_name = column_name
        self.default = default
        self.primary_key = primary_key
        self.constraints = constraints  # List of column constraints.
        self.sequence = sequence  # Name of sequence, e.g. foo_id_seq.
        self.collation = collation
        self.unindexed = unindexed
        self.choices = choices
        self.help_text = help_text
        self.verbose_name = verbose_name
        self._hidden = _hidden

        # Used internally for recovering the order in which Fields were defined
        # on the Model class.
        Field._field_counter += 1
        self._order = Field._field_counter
        self._sort_key = (self.primary_key and 1 or 2), self._order

    def __hash__(self):
        return hash(self.name + '.' + self.model.__name__)

    def bind(self, model, name, set_attribute=True):
        self.model = model
        self.name = name
        self.column_name = self.column_name or name
        if set_attribute:
            setattr(model, name, self.accessor_class(model, self, name))

    @property
    def column(self):
        return Column(self.model._meta.table, self.column_name)

    def adapt(self, value):
        return value

    def db_value(self, value):
        return value if value is None else self.adapt(value)

    def python_value(self, value):
        return value if value is None else self.adapt(value)

    def get_sort_key(self, ctx):
        return self._sort_key

    def __sql__(self, ctx):
        return ctx.sql(self.column)

    def get_modifiers(self):
        return

    def ddl_datatype(self, ctx):
        if ctx and ctx.state.field_types:
            column_type = ctx.state.field_types.get(self.field_type,
                                                    self.field_type)
        else:
            column_type = self.field_type

        modifiers = self.get_modifiers()
        if column_type and modifiers:
            modifier_literal = ', '.join([str(m) for m in modifiers])
            return SQL('%s(%s)' % (column_type, modifier_literal))
        else:
            return SQL(column_type)

    def ddl(self, ctx):
        accum = [Entity(self.column_name)]
        data_type = self.ddl_datatype(ctx)
        if data_type:
            accum.append(data_type)
        if self.unindexed:
            accum.append(SQL('UNINDEXED'))
        if not self.null:
            accum.append(SQL('NOT NULL'))
        if self.primary_key:
            accum.append(SQL('PRIMARY KEY'))
        if self.sequence:
            accum.append(SQL("DEFAULT NEXTVAL('%s')" % self.sequence))
        if self.constraints:
            accum.extend(self.constraints)
        if self.collation:
            accum.append(SQL('COLLATE %s' % self.collation))
        return NodeList(accum)


class IntegerField(Field):
    field_type = 'INT'
    adapt = int


class BigIntegerField(IntegerField):
    field_type = 'BIGINT'


class SmallIntegerField(IntegerField):
    field_type = 'SMALLINT'


class AutoField(IntegerField):
    auto_increment = True
    field_type = 'AUTO'

    def __init__(self, *args, **kwargs):
        if kwargs.get('primary_key') is False:
            raise ValueError('%s must always be a primary key.' % type(self))
        kwargs['primary_key'] = True
        super(AutoField, self).__init__(*args, **kwargs)


class BigAutoField(AutoField):
    field_type = 'BIGAUTO'


class PrimaryKeyField(AutoField):
    def __init__(self, *args, **kwargs):
        __deprecated__('"PrimaryKeyField" has been renamed to "AutoField". '
                       'Please update your code accordingly as this will be '
                       'completely removed in a subsequent release.')
        super(PrimaryKeyField, self).__init__(*args, **kwargs)


class FloatField(Field):
    field_type = 'FLOAT'
    adapt = float


class DoubleField(FloatField):
    field_type = 'DOUBLE'


class DecimalField(Field):
    field_type = 'DECIMAL'

    def __init__(self, max_digits=10, decimal_places=5, auto_round=False,
                 rounding=None, *args, **kwargs):
        self.max_digits = max_digits
        self.decimal_places = decimal_places
        self.auto_round = auto_round
        self.rounding = rounding or decimal.DefaultContext.rounding
        super(DecimalField, self).__init__(*args, **kwargs)

    def get_modifiers(self):
        return [self.max_digits, self.decimal_places]

    def db_value(self, value):
        D = decimal.Decimal
        if not value:
            return value if value is None else D(0)
        if self.auto_round:
            exp = D(10) ** (-self.decimal_places)
            rounding = self.rounding
            return D(text_type(value)).quantize(exp, rounding=rounding)
        return value

    def python_value(self, value):
        if value is not None:
            if isinstance(value, decimal.Decimal):
                return value
            return decimal.Decimal(text_type(value))


class _StringField(Field):
    def adapt(self, value):
        if isinstance(value, text_type):
            return value
        elif isinstance(value, bytes_type):
            return value.decode('utf-8')
        return text_type(value)

    def __add__(self, other): return self.concat(other)
    def __radd__(self, other): return other.concat(self)


class CharField(_StringField):
    field_type = 'VARCHAR'

    def __init__(self, max_length=255, *args, **kwargs):
        self.max_length = max_length
        super(CharField, self).__init__(*args, **kwargs)

    def get_modifiers(self):
        return self.max_length and [self.max_length] or None


class FixedCharField(CharField):
    field_type = 'CHAR'

    def python_value(self, value):
        value = super(FixedCharField, self).python_value(value)
        if value:
            value = value.strip()
        return value


class TextField(_StringField):
    field_type = 'TEXT'


class BlobField(Field):
    field_type = 'BLOB'

    def bind(self, model, name, set_attribute=True):
        self._constructor = bytearray
        if model._meta.database:
            if isinstance(model._meta.database, Proxy):
                def cb(db):
                    self._constructor = db.get_binary_type()
                model._meta.database.attach_callback(cb)
            else:
                self._constructor = model._meta.database.get_binary_type()
        return super(BlobField, self).bind(model, name, set_attribute)

    def db_value(self, value):
        if isinstance(value, text_type):
            value = value.encode('raw_unicode_escape')
        if isinstance(value, bytes_type):
            return self._constructor(value)
        return value


class BitField(BitwiseMixin, BigIntegerField):
    def __init__(self, *args, **kwargs):
        kwargs.setdefault('default', 0)
        super(BitField, self).__init__(*args, **kwargs)

    def flag(self, value):
        class FlagDescriptor(object):
            def __init__(self, field, value):
                self._field = field
                self._value = value
            def __get__(self, instance, instance_type=None):
                if instance is None:
                    return self._field.bin_and(self._value) != 0
                value = getattr(instance, self._field.name) or 0
                return (value & self._value) != 0
            def __set__(self, instance, is_set):
                if is_set not in (True, False):
                    raise ValueError('Value must be either True or False')
                value = getattr(instance, self._field.name) or 0
                if is_set:
                    value |= self._value
                else:
                    value &= ~self._value
                setattr(instance, self._field.name, value)
        return FlagDescriptor(self, value)


class BigBitFieldData(object):
    def __init__(self, instance, name):
        self.instance = instance
        self.name = name
        value = self.instance.__data__.get(self.name)
        if not value:
            value = bytearray()
        elif not isinstance(value, bytearray):
            value = bytearray(value)
        self._buffer = self.instance.__data__[self.name] = value

    def _ensure_length(self, idx):
        byte_num, byte_offset = divmod(idx, 8)
        cur_size = len(self._buffer)
        if cur_size <= byte_num:
            self._buffer.extend(b'\x00' * ((byte_num + 1) - cur_size))
        return byte_num, byte_offset

    def set_bit(self, idx):
        byte_num, byte_offset = self._ensure_length(idx)
        self._buffer[byte_num] |= (1 << byte_offset)

    def clear_bit(self, idx):
        byte_num, byte_offset = self._ensure_length(idx)
        self._buffer[byte_num] &= ~(1 << byte_offset)

    def toggle_bit(self, idx):
        byte_num, byte_offset = self._ensure_length(idx)
        self._buffer[byte_num] ^= (1 << byte_offset)
        return bool(self._buffer[byte_num] & (1 << byte_offset))

    def is_set(self, idx):
        byte_num, byte_offset = self._ensure_length(idx)
        return bool(self._buffer[byte_num] & (1 << byte_offset))

    def __repr__(self):
        return repr(self._buffer)


class BigBitFieldAccessor(FieldAccessor):
    def __get__(self, instance, instance_type=None):
        if instance is None:
            return self.field
        return BigBitFieldData(instance, self.name)
    def __set__(self, instance, value):
        if isinstance(value, memoryview):
            value = value.tobytes()
        elif isinstance(value, buffer_type):
            value = bytes(value)
        elif isinstance(value, bytearray):
            value = bytes_type(value)
        elif isinstance(value, BigBitFieldData):
            value = bytes_type(value._buffer)
        elif isinstance(value, text_type):
            value = value.encode('utf-8')
        elif not isinstance(value, bytes_type):
            raise ValueError('Value must be either a bytes, memoryview or '
                             'BigBitFieldData instance.')
        super(BigBitFieldAccessor, self).__set__(instance, value)


class BigBitField(BlobField):
    accessor_class = BigBitFieldAccessor

    def __init__(self, *args, **kwargs):
        kwargs.setdefault('default', bytes_type)
        super(BigBitField, self).__init__(*args, **kwargs)

    def db_value(self, value):
        return bytes_type(value) if value is not None else value


class UUIDField(Field):
    field_type = 'UUID'

    def db_value(self, value):
        if isinstance(value, uuid.UUID):
            return value.hex
        try:
            return uuid.UUID(value).hex
        except:
            return value

    def python_value(self, value):
        if isinstance(value, uuid.UUID):
            return value
        return None if value is None else uuid.UUID(value)


def _date_part(date_part):
    def dec(self):
        return self.model._meta.database.extract_date(date_part, self)
    return dec

def format_date_time(value, formats, post_process=None):
    post_process = post_process or (lambda x: x)
    for fmt in formats:
        try:
            return post_process(datetime.datetime.strptime(value, fmt))
        except ValueError:
            pass
    return value


class _BaseFormattedField(Field):
    formats = None

    def __init__(self, formats=None, *args, **kwargs):
        if formats is not None:
            self.formats = formats
        super(_BaseFormattedField, self).__init__(*args, **kwargs)


class DateTimeField(_BaseFormattedField):
    field_type = 'DATETIME'
    formats = [
        '%Y-%m-%d %H:%M:%S.%f',
        '%Y-%m-%d %H:%M:%S',
        '%Y-%m-%d',
    ]

    def python_value(self, value):
        if value and isinstance(value, basestring):
            return format_date_time(value, self.formats)
        return value

    year = property(_date_part('year'))
    month = property(_date_part('month'))
    day = property(_date_part('day'))
    hour = property(_date_part('hour'))
    minute = property(_date_part('minute'))
    second = property(_date_part('second'))


class DateField(_BaseFormattedField):
    field_type = 'DATE'
    formats = [
        '%Y-%m-%d',
        '%Y-%m-%d %H:%M:%S',
        '%Y-%m-%d %H:%M:%S.%f',
    ]

    def python_value(self, value):
        if value and isinstance(value, basestring):
            pp = lambda x: x.date()
            return format_date_time(value, self.formats, pp)
        elif value and isinstance(value, datetime.datetime):
            return value.date()
        return value

    year = property(_date_part('year'))
    month = property(_date_part('month'))
    day = property(_date_part('day'))


class TimeField(_BaseFormattedField):
    field_type = 'TIME'
    formats = [
        '%H:%M:%S.%f',
        '%H:%M:%S',
        '%H:%M',
        '%Y-%m-%d %H:%M:%S.%f',
        '%Y-%m-%d %H:%M:%S',
    ]

    def python_value(self, value):
        if value:
            if isinstance(value, basestring):
                pp = lambda x: x.time()
                return format_date_time(value, self.formats, pp)
            elif isinstance(value, datetime.datetime):
                return value.time()
        if value is not None and isinstance(value, datetime.timedelta):
            return (datetime.datetime.min + value).time()
        return value

    hour = property(_date_part('hour'))
    minute = property(_date_part('minute'))
    second = property(_date_part('second'))


class TimestampField(IntegerField):
    # Support second -> microsecond resolution.
    valid_resolutions = [10**i for i in range(7)]

    def __init__(self, *args, **kwargs):
        self.resolution = kwargs.pop('resolution', 1) or 1
        if self.resolution not in self.valid_resolutions:
            raise ValueError('TimestampField resolution must be one of: %s' %
                             ', '.join(str(i) for i in self.valid_resolutions))

        self.utc = kwargs.pop('utc', False) or False
        _dt = datetime.datetime
        self._conv = _dt.utcfromtimestamp if self.utc else _dt.fromtimestamp
        _default = _dt.utcnow if self.utc else _dt.now
        kwargs.setdefault('default', _default)
        super(TimestampField, self).__init__(*args, **kwargs)

    def db_value(self, value):
        if value is None:
            return

        if isinstance(value, datetime.datetime):
            pass
        elif isinstance(value, datetime.date):
            value = datetime.datetime(value.year, value.month, value.day)
        else:
            return int(round(value * self.resolution))

        if self.utc:
            timestamp = calendar.timegm(value.utctimetuple())
        else:
            timestamp = time.mktime(value.timetuple())
        timestamp += (value.microsecond * .000001)
        if self.resolution > 1:
            timestamp *= self.resolution
        return int(round(timestamp))

    def python_value(self, value):
        if value is not None and isinstance(value, (int, float, long)):
            if value == 0:
                return
            elif self.resolution > 1:
                ticks_to_microsecond = 1000000 // self.resolution
                value, ticks = divmod(value, self.resolution)
                microseconds = int(ticks * ticks_to_microsecond)
                return self._conv(value).replace(microsecond=microseconds)
            else:
                return self._conv(value)
        return value


class IPField(BigIntegerField):
    def db_value(self, val):
        if val is not None:
            return struct.unpack('!I', socket.inet_aton(val))[0]

    def python_value(self, val):
        if val is not None:
            return socket.inet_ntoa(struct.pack('!I', val))


class BooleanField(Field):
    field_type = 'BOOL'
    adapt = bool


class BareField(Field):
    def __init__(self, adapt=None, *args, **kwargs):
        super(BareField, self).__init__(*args, **kwargs)
        if adapt is not None:
            self.adapt = adapt

    def ddl(self, ctx):
        return Entity(self.column_name)


class ForeignKeyField(Field):
    accessor_class = ForeignKeyAccessor

    def __init__(self, model, field=None, backref=None, on_delete=None,
                 on_update=None, _deferred=None, rel_model=None, to_field=None,
                 object_id_name=None, related_name=None, *args, **kwargs):
        super(ForeignKeyField, self).__init__(*args, **kwargs)
        if rel_model is not None:
            __deprecated__('"rel_model" has been deprecated in favor of '
                           '"model" for ForeignKeyField objects.')
            model = rel_model
        if to_field is not None:
            __deprecated__('"to_field" has been deprecated in favor of '
                           '"field" for ForeignKeyField objects.')
            field = to_field
        if related_name is not None:
            __deprecated__('"related_name" has been deprecated in favor of '
                           '"backref" for Field objects.')
            backref = related_name

        self.rel_model = model
        self.rel_field = field
        self.declared_backref = backref
        self.backref = None
        self.on_delete = on_delete
        self.on_update = on_update
        self.deferred = _deferred
        self.object_id_name = object_id_name

    def __repr__(self):
        if hasattr(self, 'model') and getattr(self, 'name', None):
            return '<%s: "%s"."%s">' % (type(self).__name__,
                                        self.model._meta.name,
                                        self.name)
        return '<%s: (unbound)>' % type(self).__name__

    @property
    def field_type(self):
        if not isinstance(self.rel_field, AutoField):
            return self.rel_field.field_type
        return IntegerField.field_type

    def get_modifiers(self):
        if not isinstance(self.rel_field, AutoField):
            return self.rel_field.get_modifiers()
        return super(ForeignKeyField, self).get_modifiers()

    def adapt(self, value):
        return self.rel_field.adapt(value)

    def db_value(self, value):
        if isinstance(value, self.rel_model):
            value = value.get_id()
        return self.rel_field.db_value(value)

    def python_value(self, value):
        if isinstance(value, self.rel_model):
            return value
        return self.rel_field.python_value(value)

    def expression(self):
        return self.column == self.rel_field.column

    def bind(self, model, name, set_attribute=True):
        if not self.column_name:
            self.column_name = name if name.endswith('_id') else name + '_id'
        if not self.object_id_name:
            self.object_id_name = self.column_name
            if self.object_id_name == name:
                self.object_id_name += '_id'
        elif self.object_id_name == name:
            raise ValueError('ForeignKeyField "%s"."%s" specifies an '
                             'object_id_name that conflicts with its field '
                             'name.' % (model._meta.name, name))
        if self.rel_model == 'self':
            self.rel_model = model
        if isinstance(self.rel_field, basestring):
            self.rel_field = getattr(self.rel_model, self.rel_field)
        elif self.rel_field is None:
            self.rel_field = self.rel_model._meta.primary_key

        # Bind field before assigning backref, so field is bound when
        # calling declared_backref() (if callable).
        super(ForeignKeyField, self).bind(model, name, set_attribute)

        if callable(self.declared_backref):
            self.backref = self.declared_backref(self)
        else:
            self.backref, self.declared_backref = self.declared_backref, None
        if not self.backref:
            self.backref = '%s_set' % model._meta.name

        if set_attribute:
            setattr(model, self.object_id_name, ObjectIdAccessor(self))
            if self.backref not in '!+':
                setattr(self.rel_model, self.backref, BackrefAccessor(self))

    def foreign_key_constraint(self):
        parts = [
            SQL('FOREIGN KEY'),
            EnclosedNodeList((self,)),
            SQL('REFERENCES'),
            self.rel_model,
            EnclosedNodeList((self.rel_field,))]
        if self.on_delete:
            parts.append(SQL('ON DELETE %s' % self.on_delete))
        if self.on_update:
            parts.append(SQL('ON UPDATE %s' % self.on_update))
        return NodeList(parts)

    def __getattr__(self, attr):
        if attr.startswith('__'):
            # Prevent recursion error when deep-copying.
            raise AttributeError('Cannot look-up non-existant "__" methods.')
        if attr in self.rel_model._meta.fields:
            return self.rel_model._meta.fields[attr]
        raise AttributeError('%r has no attribute %s, nor is it a valid field '
                             'on %s.' % (self, attr, self.rel_model))


class DeferredForeignKey(Field):
    _unresolved = set()

    def __init__(self, rel_model_name, **kwargs):
        self.field_kwargs = kwargs
        self.rel_model_name = rel_model_name.lower()
        DeferredForeignKey._unresolved.add(self)
        super(DeferredForeignKey, self).__init__()

    __hash__ = object.__hash__

    def set_model(self, rel_model):
        field = ForeignKeyField(rel_model, _deferred=True, **self.field_kwargs)
        self.model._meta.add_field(self.name, field)

    @staticmethod
    def resolve(model_cls):
        unresolved = list(DeferredForeignKey._unresolved)
        for dr in unresolved:
            if dr.rel_model_name == model_cls.__name__.lower():
                dr.set_model(model_cls)
                DeferredForeignKey._unresolved.discard(dr)


class DeferredThroughModel(object):
    def set_field(self, model, field, name):
        self.model = model
        self.field = field
        self.name = name

    def set_model(self, through_model):
        self.field.through_model = through_model
        self.field.bind(self.model, self.name)


class MetaField(Field):
    column_name = default = model = name = None
    primary_key = False


class ManyToManyFieldAccessor(FieldAccessor):
    def __init__(self, model, field, name):
        super(ManyToManyFieldAccessor, self).__init__(model, field, name)
        self.model = field.model
        self.rel_model = field.rel_model
        self.through_model = field.get_through_model()
        self.src_fk = self.through_model._meta.model_refs[self.model][0]
        self.dest_fk = self.through_model._meta.model_refs[self.rel_model][0]

    def __get__(self, instance, instance_type=None, force_query=False):
        if instance is not None:
            if not force_query and isinstance(getattr(instance, self.src_fk.backref), list):
                return [getattr(obj, self.dest_fk.name) for obj in getattr(instance, self.src_fk.backref)]
            else:
                return (ManyToManyQuery(instance, self, self.rel_model)
                        .join(self.through_model)
                        .join(self.model)
                        .where(self.src_fk == instance))
        return self.field

    def __set__(self, instance, value):
        query = self.__get__(instance, force_query=True)
        query.add(value, clear_existing=True)


class ManyToManyField(MetaField):
    accessor_class = ManyToManyFieldAccessor

    def __init__(self, model, backref=None, through_model=None,
                 _is_backref=False):
        if through_model is not None and not (
                isinstance(through_model, DeferredThroughModel) or
                is_model(through_model)):
            raise TypeError('Unexpected value for through_model. Expected '
                            'Model or DeferredThroughModel.')
        self.rel_model = model
        self.backref = backref
        self.through_model = through_model
        self._is_backref = _is_backref

    def _get_descriptor(self):
        return ManyToManyFieldAccessor(self)

    def bind(self, model, name, set_attribute=True):
        if isinstance(self.through_model, DeferredThroughModel):
            self.through_model.set_field(model, self, name)
            return

        super(ManyToManyField, self).bind(model, name, set_attribute)

        if not self._is_backref:
            many_to_many_field = ManyToManyField(
                self.model,
                through_model=self.through_model,
                _is_backref=True)
            backref = self.backref or model._meta.name + 's'
            self.rel_model._meta.add_field(backref, many_to_many_field)

    def get_models(self):
        return [model for _, model in sorted((
            (self._is_backref, self.model),
            (not self._is_backref, self.rel_model)))]

    def get_through_model(self):
        if not self.through_model:
            lhs, rhs = self.get_models()
            tables = [model._meta.table_name for model in (lhs, rhs)]

            class Meta:
                database = self.model._meta.database
                table_name = '%s_%s_through' % tuple(tables)
                indexes = (
                    ((lhs._meta.name, rhs._meta.name),
                     True),)

            attrs = {
                lhs._meta.name: ForeignKeyField(lhs),
                rhs._meta.name: ForeignKeyField(rhs)}
            attrs['Meta'] = Meta

            self.through_model = type(
                '%s%sThrough' % (lhs.__name__, rhs.__name__),
                (Model,),
                attrs)

        return self.through_model


class VirtualField(MetaField):
    field_class = None

    def __init__(self, field_class=None, *args, **kwargs):
        Field = field_class if field_class is not None else self.field_class
        self.field_instance = Field() if Field is not None else None
        super(VirtualField, self).__init__(*args, **kwargs)

    def db_value(self, value):
        if self.field_instance is not None:
            return self.field_instance.db_value(value)
        return value

    def python_value(self, value):
        if self.field_instance is not None:
            return self.field_instance.python_value(value)
        return value

    def bind(self, model, name, set_attribute=True):
        self.model = model
        self.column_name = self.name = name
        setattr(model, name, self.accessor_class(model, self, name))


class CompositeKey(MetaField):
    sequence = None

    def __init__(self, *field_names):
        self.field_names = field_names

    def __get__(self, instance, instance_type=None):
        if instance is not None:
            return tuple([getattr(instance, field_name)
                          for field_name in self.field_names])
        return self

    def __set__(self, instance, value):
        if not isinstance(value, (list, tuple)):
            raise TypeError('A list or tuple must be used to set the value of '
                            'a composite primary key.')
        if len(value) != len(self.field_names):
            raise ValueError('The length of the value must equal the number '
                             'of columns of the composite primary key.')
        for idx, field_value in enumerate(value):
            setattr(instance, self.field_names[idx], field_value)

    def __eq__(self, other):
        expressions = [(self.model._meta.fields[field] == value)
                       for field, value in zip(self.field_names, other)]
        return reduce(operator.and_, expressions)

    def __ne__(self, other):
        return ~(self == other)

    def __hash__(self):
        return hash((self.model.__name__, self.field_names))

    def __sql__(self, ctx):
        return ctx.sql(CommaNodeList([self.model._meta.fields[field]
                                      for field in self.field_names]))

    def bind(self, model, name, set_attribute=True):
        self.model = model
        self.column_name = self.name = name
        setattr(model, self.name, self)


class _SortedFieldList(object):
    __slots__ = ('_keys', '_items')

    def __init__(self):
        self._keys = []
        self._items = []

    def __getitem__(self, i):
        return self._items[i]

    def __iter__(self):
        return iter(self._items)

    def __contains__(self, item):
        k = item._sort_key
        i = bisect_left(self._keys, k)
        j = bisect_right(self._keys, k)
        return item in self._items[i:j]

    def index(self, field):
        return self._keys.index(field._sort_key)

    def insert(self, item):
        k = item._sort_key
        i = bisect_left(self._keys, k)
        self._keys.insert(i, k)
        self._items.insert(i, item)

    def remove(self, item):
        idx = self.index(item)
        del self._items[idx]
        del self._keys[idx]


# MODELS


class SchemaManager(object):
    def __init__(self, model, database=None, **context_options):
        self.model = model
        self._database = database
        context_options.setdefault('scope', SCOPE_VALUES)
        self.context_options = context_options

    @property
    def database(self):
        return self._database or self.model._meta.database

    @database.setter
    def database(self, value):
        self._database = value

    def _create_context(self):
        return self.database.get_sql_context(**self.context_options)

    def _create_table(self, safe=True, **options):
        is_temp = options.pop('temporary', False)
        ctx = self._create_context()
        ctx.literal('CREATE TEMPORARY TABLE ' if is_temp else 'CREATE TABLE ')
        if safe:
            ctx.literal('IF NOT EXISTS ')
        ctx.sql(self.model).literal(' ')

        columns = []
        constraints = []
        meta = self.model._meta
        if meta.composite_key:
            pk_columns = [meta.fields[field_name].column
                          for field_name in meta.primary_key.field_names]
            constraints.append(NodeList((SQL('PRIMARY KEY'),
                                         EnclosedNodeList(pk_columns))))

        for field in meta.sorted_fields:
            columns.append(field.ddl(ctx))
            if isinstance(field, ForeignKeyField) and not field.deferred:
                constraints.append(field.foreign_key_constraint())

        if meta.constraints:
            constraints.extend(meta.constraints)

        constraints.extend(self._create_table_option_sql(options))
        ctx.sql(EnclosedNodeList(columns + constraints))

        if meta.without_rowid:
            ctx.literal(' WITHOUT ROWID')
        return ctx

    def _create_table_option_sql(self, options):
        accum = []
        options = merge_dict(self.model._meta.options or {}, options)
        if not options:
            return accum

        for key, value in sorted(options.items()):
            if not isinstance(value, Node):
                if is_model(value):
                    value = value._meta.table
                else:
                    value = SQL(value)
            accum.append(NodeList((SQL(key), value), glue='='))
        return accum

    def create_table(self, safe=True, **options):
        self.database.execute(self._create_table(safe=safe, **options))

    def _drop_table(self, safe=True, **options):
        is_temp = options.pop('temporary', False)
        ctx = (self._create_context()
               .literal('DROP TEMPORARY ' if is_temp else 'DROP ')
               .literal('TABLE IF EXISTS ' if safe else 'TABLE ')
               .sql(self.model))
        if options.get('cascade'):
            ctx = ctx.literal(' CASCADE')
        return ctx

    def drop_table(self, safe=True, **options):
        self.database.execute(self._drop_table(safe=safe, **options))

    def _create_indexes(self, safe=True):
        return [self._create_index(index, safe)
                for index in self.model._meta.fields_to_index()]

    def _create_index(self, index, safe=True):
        if isinstance(index, Index):
            if not self.database.safe_create_index:
                index = index.safe(False)
            elif index._safe != safe:
                index = index.safe(safe)
        return self._create_context().sql(index)

    def create_indexes(self, safe=True):
        for query in self._create_indexes(safe=safe):
            self.database.execute(query)

    def _drop_indexes(self, safe=True):
        return [self._drop_index(index, safe)
                for index in self.model._meta.fields_to_index()
                if isinstance(index, Index)]

    def _drop_index(self, index, safe):
        statement = 'DROP INDEX '
        if safe and self.database.safe_drop_index:
            statement += 'IF EXISTS '
        return (self
                ._create_context()
                .literal(statement)
                .sql(Entity(index._name)))

    def drop_indexes(self, safe=True):
        for query in self._drop_indexes(safe=safe):
            self.database.execute(query)

    def _check_sequences(self, field):
        if not field.sequence or not self.database.sequences:
            raise ValueError('Sequences are either not supported, or are not '
                             'defined for "%s".' % field.name)

    def _create_sequence(self, field):
        self._check_sequences(field)
        if not self.database.sequence_exists(field.sequence):
            return (self
                    ._create_context()
                    .literal('CREATE SEQUENCE ')
                    .sql(Entity(field.sequence)))

    def create_sequence(self, field):
        self.database.execute(self._create_sequence(field))

    def _drop_sequence(self, field):
        self._check_sequences(field)
        if self.database.sequence_exists(field.sequence):
            return (self
                    ._create_context()
                    .literal('DROP SEQUENCE ')
                    .sql(Entity(field.sequence)))

    def drop_sequence(self, field):
        self.database.execute(self._drop_sequence(field))

    def _create_foreign_key(self, field):
        name = 'fk_%s_%s_refs_%s' % (field.model._meta.table_name,
                                     field.column_name,
                                     field.rel_model._meta.table_name)
        return (self
                ._create_context()
                .literal('ALTER TABLE ')
                .sql(field.model)
                .literal(' ADD CONSTRAINT ')
                .sql(Entity(name))
                .literal(' ')
                .sql(field.foreign_key_constraint()))

    def create_foreign_key(self, field):
        self.database.execute(self._create_foreign_key(field))

    def create_all(self, safe=True, **table_options):
        if self.database.sequences:
            for field in self.model._meta.sorted_fields:
                if field and field.sequence:
                    self.create_sequence(field)

        self.create_table(safe, **table_options)
        self.create_indexes(safe=safe)

    def drop_all(self, safe=True, **options):
        self.drop_table(safe, **options)


class Metadata(object):
    def __init__(self, model, database=None, table_name=None, indexes=None,
                 primary_key=None, constraints=None, schema=None,
                 only_save_dirty=False, table_alias=None, depends_on=None,
                 options=None, db_table=None, table_function=None,
                 without_rowid=False, **kwargs):
        if db_table is not None:
            __deprecated__('"db_table" has been deprecated in favor of '
                           '"table_name" for Models.')
            table_name = db_table
        self.model = model
        self.database = database

        self.fields = {}
        self.columns = {}
        self.combined = {}

        self._sorted_field_list = _SortedFieldList()
        self.sorted_fields = []
        self.sorted_field_names = []

        self.defaults = {}
        self._default_by_name = {}
        self._default_dict = {}
        self._default_callables = {}
        self._default_callable_list = []

        self.name = model.__name__.lower()
        self.table_function = table_function
        if not table_name:
            table_name = (self.table_function(model)
                          if self.table_function
                          else re.sub('[^\w]+', '_', self.name))
        self.table_name = table_name
        self._table = None

        self.indexes = list(indexes) if indexes else []
        self.constraints = constraints
        self._schema = schema
        self.primary_key = primary_key
        self.composite_key = self.auto_increment = None
        self.only_save_dirty = only_save_dirty
        self.table_alias = table_alias
        self.depends_on = depends_on
        self.without_rowid = without_rowid

        self.refs = {}
        self.backrefs = {}
        self.model_refs = collections.defaultdict(list)
        self.model_backrefs = collections.defaultdict(list)
        self.manytomany = {}

        self.options = options or {}
        for key, value in kwargs.items():
            setattr(self, key, value)
        self._additional_keys = set(kwargs.keys())

    def model_graph(self, refs=True, backrefs=True, depth_first=True):
        if not refs and not backrefs:
            raise ValueError('One of `refs` or `backrefs` must be True.')

        accum = [(None, self.model, None)]
        seen = set()
        queue = collections.deque((self,))
        method = queue.pop if depth_first else queue.popleft

        while queue:
            curr = method()
            if curr in seen: continue
            seen.add(curr)

            if refs:
                for fk, model in curr.refs.items():
                    accum.append((fk, model, False))
                    queue.append(model._meta)
            if backrefs:
                for fk, model in curr.backrefs.items():
                    accum.append((fk, model, True))
                    queue.append(model._meta)

        return accum

    def add_ref(self, field):
        rel = field.rel_model
        self.refs[field] = rel
        self.model_refs[rel].append(field)
        rel._meta.backrefs[field] = self.model
        rel._meta.model_backrefs[self.model].append(field)

    def remove_ref(self, field):
        rel = field.rel_model
        del self.refs[field]
        self.model_ref[rel].remove(field)
        del rel._meta.backrefs[field]
        rel._meta.model_backrefs[self.model].remove(field)

    def add_manytomany(self, field):
        self.manytomany[field.name] = field

    def remove_manytomany(self, field):
        del self.manytomany[field.name]

    @property
    def table(self):
        if self._table is None:
            self._table = Table(
                self.table_name,
                [field.column_name for field in self.sorted_fields],
                schema=self.schema,
                alias=self.table_alias,
                _model=self.model,
                _database=self.database)
        return self._table

    @table.setter
    def table(self, value):
        raise AttributeError('Cannot set the "table".')

    @table.deleter
    def table(self):
        self._table = None

    @property
    def schema(self):
        return self._schema

    @schema.setter
    def schema(self, value):
        self._schema = value
        del self.table

    @property
    def entity(self):
        if self._schema:
            return Entity(self._schema, self.table_name)
        else:
            return Entity(self.table_name)

    def _update_sorted_fields(self):
        self.sorted_fields = list(self._sorted_field_list)
        self.sorted_field_names = [f.name for f in self.sorted_fields]

    def add_field(self, field_name, field, set_attribute=True):
        if field_name in self.fields:
            self.remove_field(field_name)
        elif field_name in self.manytomany:
            self.remove_manytomany(self.manytomany[field_name])

        if not isinstance(field, MetaField):
            del self.table
            field.bind(self.model, field_name, set_attribute)
            self.fields[field.name] = field
            self.columns[field.column_name] = field
            self.combined[field.name] = field
            self.combined[field.column_name] = field

            self._sorted_field_list.insert(field)
            self._update_sorted_fields()

            if field.default is not None:
                # This optimization helps speed up model instance construction.
                self.defaults[field] = field.default
                if callable(field.default):
                    self._default_callables[field] = field.default
                    self._default_callable_list.append((field.name,
                                                        field.default))
                else:
                    self._default_dict[field] = field.default
                    self._default_by_name[field.name] = field.default
        else:
            field.bind(self.model, field_name, set_attribute)

        if isinstance(field, ForeignKeyField):
            self.add_ref(field)
        elif isinstance(field, ManyToManyField):
            self.add_manytomany(field)

    def remove_field(self, field_name):
        if field_name not in self.fields:
            return

        del self.table
        original = self.fields.pop(field_name)
        del self.columns[original.column_name]
        del self.combined[field_name]
        try:
            del self.combined[original.column_name]
        except KeyError:
            pass
        self._sorted_field_list.remove(original)
        self._update_sorted_fields()

        if original.default is not None:
            del self.defaults[original]
            if self._default_callables.pop(original, None):
                for i, (name, _) in enumerate(self._default_callable_list):
                    if name == field_name:
                        self._default_callable_list.pop(i)
                        break
            else:
                self._default_dict.pop(original, None)
                self._default_by_name.pop(original.name, None)

        if isinstance(original, ForeignKeyField):
            self.remove_ref(original)

    def set_primary_key(self, name, field):
        self.composite_key = isinstance(field, CompositeKey)
        self.add_field(name, field)
        self.primary_key = field
        self.auto_increment = (
            field.auto_increment or
            bool(field.sequence))

    def get_primary_keys(self):
        if self.composite_key:
            return tuple([self.fields[field_name]
                          for field_name in self.primary_key.field_names])
        else:
            return (self.primary_key,) if self.primary_key is not False else ()

    def get_default_dict(self):
        dd = self._default_by_name.copy()
        for field_name, default in self._default_callable_list:
            dd[field_name] = default()
        return dd

    def fields_to_index(self):
        indexes = []
        for f in self.sorted_fields:
            if f.primary_key:
                continue
            if f.index or f.unique or isinstance(f, ForeignKeyField):
                indexes.append(ModelIndex(self.model, (f,), unique=f.unique))

        for index_obj in self.indexes:
            if isinstance(index_obj, Node):
                indexes.append(index_obj)
            elif isinstance(index_obj, (list, tuple)):
                index_parts, unique = index_obj
                fields = []
                for part in index_parts:
                    if isinstance(part, basestring):
                        fields.append(self.combined[part])
                    elif isinstance(part, Node):
                        fields.append(part)
                    else:
                        raise ValueError('Expected either a field name or a '
                                         'subclass of Node. Got: %s' % part)
                indexes.append(ModelIndex(self.model, fields, unique=unique))

        return indexes

    def set_database(self, database):
        self.database = database
        self.model._schema._database = database


class SubclassAwareMetadata(Metadata):
    models = []

    def __init__(self, model, *args, **kwargs):
        super(SubclassAwareMetadata, self).__init__(model, *args, **kwargs)
        self.models.append(model)

    def map_models(self, fn):
        for model in self.models:
            fn(model)


class DoesNotExist(Exception): pass


class ModelBase(type):
    inheritable = set(['constraints', 'database', 'indexes', 'primary_key',
                       'options', 'schema', 'table_function',
                       'only_save_dirty'])

    def __new__(cls, name, bases, attrs):
        if name == MODEL_BASE or bases[0].__name__ == MODEL_BASE:
            return super(ModelBase, cls).__new__(cls, name, bases, attrs)

        meta_options = {}
        meta = attrs.pop('Meta', None)
        if meta:
            for k, v in meta.__dict__.items():
                if not k.startswith('_'):
                    meta_options[k] = v

        pk = getattr(meta, 'primary_key', None)
        pk_name = parent_pk = None

        # Inherit any field descriptors by deep copying the underlying field
        # into the attrs of the new model, additionally see if the bases define
        # inheritable model options and swipe them.
        for b in bases:
            if not hasattr(b, '_meta'):
                continue

            base_meta = b._meta
            if parent_pk is None:
                parent_pk = deepcopy(base_meta.primary_key)
            all_inheritable = cls.inheritable | base_meta._additional_keys
            for k in base_meta.__dict__:
                if k in all_inheritable and k not in meta_options:
                    meta_options[k] = base_meta.__dict__[k]
            meta_options.setdefault('schema', base_meta.schema)

            for (k, v) in b.__dict__.items():
                if k in attrs: continue

                if isinstance(v, FieldAccessor) and not v.field.primary_key:
                    attrs[k] = deepcopy(v.field)

        sopts = meta_options.pop('schema_options', None) or {}
        Meta = meta_options.get('model_metadata_class', Metadata)
        Schema = meta_options.get('schema_manager_class', SchemaManager)

        # Construct the new class.
        cls = super(ModelBase, cls).__new__(cls, name, bases, attrs)
        cls.__data__ = cls.__rel__ = None

        cls._meta = Meta(cls, **meta_options)
        cls._schema = Schema(cls, **sopts)

        fields = []
        for key, value in cls.__dict__.items():
            if isinstance(value, Field):
                if value.primary_key and pk:
                    raise ValueError('over-determined primary key %s.' % name)
                elif value.primary_key:
                    pk, pk_name = value, key
                else:
                    fields.append((key, value))

        if pk is None:
            if parent_pk is not False:
                pk, pk_name = ((parent_pk, parent_pk.name)
                               if parent_pk is not None else
                               (AutoField(), 'id'))
            else:
                pk = False
        elif isinstance(pk, CompositeKey):
            pk_name = '__composite_key__'
            cls._meta.composite_key = True

        if pk is not False:
            cls._meta.set_primary_key(pk_name, pk)

        for name, field in fields:
            cls._meta.add_field(name, field)

        # Create a repr and error class before finalizing.
        if hasattr(cls, '__unicode__'):
            setattr(cls, '__repr__', lambda self: '<%s: %r>' % (
                cls.__name__, self.__unicode__()))

        exc_name = '%sDoesNotExist' % cls.__name__
        exc_attrs = {'__module__': cls.__module__}
        exception_class = type(exc_name, (DoesNotExist,), exc_attrs)
        cls.DoesNotExist = exception_class

        # Call validation hook, allowing additional model validation.
        cls.validate_model()
        DeferredForeignKey.resolve(cls)
        return cls

    def __iter__(self):
        return iter(self.select())

    def __getitem__(self, key):
        return self.get_by_id(key)

    def __setitem__(self, key, value):
        self.set_by_id(key, value)

    def __delitem__(self, key):
        self.delete_by_id(key)

    def __contains__(self, key):
        try:
            self.get_by_id(key)
        except self.DoesNotExist:
            return False
        else:
            return True

    def __len__(self):
        return self.select().count()
    def __bool__(self): return True
    __nonzero__ = __bool__  # Python 2.


class _BoundModelsContext(_callable_context_manager):
    def __init__(self, models, database, bind_refs, bind_backrefs):
        self.models = models
        self.database = database
        self.bind_refs = bind_refs
        self.bind_backrefs = bind_backrefs

    def __enter__(self):
        self._orig_database = []
        for model in self.models:
            self._orig_database.append(model._meta.database)
            model.bind(self.database, self.bind_refs, self.bind_backrefs)
        return self.models

    def __exit__(self, exc_type, exc_val, exc_tb):
        for model, db in zip(self.models, self._orig_database):
            model.bind(db, self.bind_refs, self.bind_backrefs)


class Model(with_metaclass(ModelBase, Node)):
    def __init__(self, *args, **kwargs):
        if kwargs.pop('__no_default__', None):
            self.__data__ = {}
        else:
            self.__data__ = self._meta.get_default_dict()
        self._dirty = set(self.__data__)
        self.__rel__ = {}

        for k in kwargs:
            setattr(self, k, kwargs[k])

    @classmethod
    def validate_model(cls):
        pass

    @classmethod
    def alias(cls, alias=None):
        return ModelAlias(cls, alias)

    @classmethod
    def select(cls, *fields):
        is_default = not fields
        if not fields:
            fields = cls._meta.sorted_fields
        return ModelSelect(cls, fields, is_default=is_default)

    @classmethod
    def _normalize_data(cls, data, kwargs):
        normalized = {}
        if data:
            if not isinstance(data, dict):
                if kwargs:
                    raise ValueError('Data cannot be mixed with keyword '
                                     'arguments: %s' % data)
                return data
            for key in data:
                try:
                    field = (key if isinstance(key, Field)
                             else cls._meta.combined[key])
                except KeyError:
                    raise ValueError('Unrecognized field name: "%s" in %s.' %
                                     (key, data))
                normalized[field] = data[key]
        if kwargs:
            for key in kwargs:
                try:
                    normalized[cls._meta.combined[key]] = kwargs[key]
                except KeyError:
                    normalized[getattr(cls, key)] = kwargs[key]
        return normalized

    @classmethod
    def update(cls, __data=None, **update):
        return ModelUpdate(cls, cls._normalize_data(__data, update))

    @classmethod
    def insert(cls, __data=None, **insert):
        return ModelInsert(cls, cls._normalize_data(__data, insert))

    @classmethod
    def insert_many(cls, rows, fields=None):
        return ModelInsert(cls, insert=rows, columns=fields)

    @classmethod
    def insert_from(cls, query, fields):
        columns = [getattr(cls, field) if isinstance(field, basestring)
                   else field for field in fields]
        return ModelInsert(cls, insert=query, columns=columns)

    @classmethod
    def replace(cls, __data=None, **insert):
        return cls.insert(__data, **insert).on_conflict('REPLACE')

    @classmethod
    def replace_many(cls, rows, fields=None):
        return (cls
                .insert_many(rows=rows, fields=fields)
                .on_conflict('REPLACE'))

    @classmethod
    def raw(cls, sql, *params):
        return ModelRaw(cls, sql, params)

    @classmethod
    def delete(cls):
        return ModelDelete(cls)

    @classmethod
    def create(cls, **query):
        inst = cls(**query)
        inst.save(force_insert=True)
        return inst

    @classmethod
    def noop(cls):
        return NoopModelSelect(cls, ())

    @classmethod
    def get(cls, *query, **filters):
        sq = cls.select()
        if query:
            sq = sq.where(*query)
        if filters:
            sq = sq.filter(**filters)
        return sq.get()

    @classmethod
    def get_or_none(cls, *query, **filters):
        try:
            return cls.get(*query, **filters)
        except DoesNotExist:
            pass

    @classmethod
    def get_by_id(cls, pk):
        return cls.get(cls._meta.primary_key == pk)

    @classmethod
    def set_by_id(cls, key, value):
        if key is None:
            return cls.insert(value).execute()
        else:
            return (cls.update(value)
                    .where(cls._meta.primary_key == key).execute())

    @classmethod
    def delete_by_id(cls, pk):
        return cls.delete().where(cls._meta.primary_key == pk).execute()

    @classmethod
    def get_or_create(cls, **kwargs):
        defaults = kwargs.pop('defaults', {})
        query = cls.select()
        for field, value in kwargs.items():
            query = query.where(getattr(cls, field) == value)

        try:
            return query.get(), False
        except cls.DoesNotExist:
            try:
                if defaults:
                    kwargs.update(defaults)
                with cls._meta.database.atomic():
                    return cls.create(**kwargs), True
            except IntegrityError as exc:
                try:
                    return query.get(), False
                except cls.DoesNotExist:
                    raise exc

    @classmethod
    def filter(cls, *dq_nodes, **filters):
        return cls.select().filter(*dq_nodes, **filters)

    def get_id(self):
        return getattr(self, self._meta.primary_key.name)

    _pk = property(get_id)

    @_pk.setter
    def _pk(self, value):
        setattr(self, self._meta.primary_key.name, value)

    def _pk_expr(self):
        return self._meta.primary_key == self._pk

    def _prune_fields(self, field_dict, only):
        new_data = {}
        for field in only:
            if isinstance(field, basestring):
                field = self._meta.combined[field]
            if field.name in field_dict:
                new_data[field.name] = field_dict[field.name]
        return new_data

    def _populate_unsaved_relations(self, field_dict):
        for foreign_key_field in self._meta.refs:
            foreign_key = foreign_key_field.name
            conditions = (
                foreign_key in field_dict and
                field_dict[foreign_key] is None and
                self.__rel__.get(foreign_key) is not None)
            if conditions:
                setattr(self, foreign_key, getattr(self, foreign_key))
                field_dict[foreign_key] = self.__data__[foreign_key]

    def save(self, force_insert=False, only=None):
        field_dict = self.__data__.copy()
        if self._meta.primary_key is not False:
            pk_field = self._meta.primary_key
            pk_value = self._pk
        else:
            pk_field = pk_value = None
        if only:
            field_dict = self._prune_fields(field_dict, only)
        elif self._meta.only_save_dirty and not force_insert:
            field_dict = self._prune_fields(field_dict, self.dirty_fields)
            if not field_dict:
                self._dirty.clear()
                return False

        self._populate_unsaved_relations(field_dict)
        if pk_value is not None and not force_insert:
            if self._meta.composite_key:
                for pk_part_name in pk_field.field_names:
                    field_dict.pop(pk_part_name, None)
            else:
                field_dict.pop(pk_field.name, None)
            rows = self.update(**field_dict).where(self._pk_expr()).execute()
        elif pk_field is None or not self._meta.auto_increment:
            self.insert(**field_dict).execute()
            rows = 1
        else:
            pk_from_cursor = self.insert(**field_dict).execute()
            if pk_from_cursor is not None:
                pk_value = pk_from_cursor
            self._pk = pk_value
            rows = 1
        self._dirty.clear()
        return rows

    def is_dirty(self):
        return bool(self._dirty)

    @property
    def dirty_fields(self):
        return [f for f in self._meta.sorted_fields if f.name in self._dirty]

    def dependencies(self, search_nullable=False):
        model_class = type(self)
        query = self.select(self._meta.primary_key).where(self._pk_expr())
        stack = [(type(self), query)]
        seen = set()

        while stack:
            klass, query = stack.pop()
            if klass in seen:
                continue
            seen.add(klass)
            for fk, rel_model in klass._meta.backrefs.items():
                if rel_model is model_class:
                    node = (fk == self.__data__[fk.rel_field.name])
                else:
                    node = fk << query
                subquery = (rel_model.select(rel_model._meta.primary_key)
                            .where(node))
                if not fk.null or search_nullable:
                    stack.append((rel_model, subquery))
                yield (node, fk)

    def delete_instance(self, recursive=False, delete_nullable=False):
        if recursive:
            dependencies = self.dependencies(delete_nullable)
            for query, fk in reversed(list(dependencies)):
                model = fk.model
                if fk.null and not delete_nullable:
                    model.update(**{fk.name: None}).where(query).execute()
                else:
                    model.delete().where(query).execute()
        return self.delete().where(self._pk_expr()).execute()

    def __hash__(self):
        return hash((self.__class__, self._pk))

    def __eq__(self, other):
        return (
            other.__class__ == self.__class__ and
            self._pk is not None and
            other._pk == self._pk)

    def __ne__(self, other):
        return not self == other

    def __sql__(self, ctx):
        return ctx.sql(getattr(self, self._meta.primary_key.name))

    @classmethod
    def bind(cls, database, bind_refs=True, bind_backrefs=True):
        is_different = cls._meta.database is not database
        cls._meta.set_database(database)
        if bind_refs or bind_backrefs:
            G = cls._meta.model_graph(refs=bind_refs, backrefs=bind_backrefs)
            for _, model, is_backref in G:
                model._meta.set_database(database)
        return is_different

    @classmethod
    def bind_ctx(cls, database, bind_refs=True, bind_backrefs=True):
        return _BoundModelsContext((cls,), database, bind_refs, bind_backrefs)

    @classmethod
    def table_exists(cls):
        meta = cls._meta
        return meta.database.table_exists(meta.table, meta.schema)

    @classmethod
    def create_table(cls, safe=True, **options):
        if 'fail_silently' in options:
            __deprecated__('"fail_silently" has been deprecated in favor of '
                           '"safe" for the create_table() method.')
            safe = options.pop('fail_silently')

        if safe and not cls._meta.database.safe_create_index \
           and cls.table_exists():
            return
        cls._schema.create_all(safe, **options)

    @classmethod
    def drop_table(cls, safe=True, **options):
        if safe and not cls._meta.database.safe_drop_index \
           and not cls.table_exists():
            return
        cls._schema.drop_all(safe, **options)

    @classmethod
    def index(cls, *fields, **kwargs):
        return ModelIndex(cls, fields, **kwargs)

    @classmethod
    def add_index(cls, *fields, **kwargs):
        if len(fields) == 1 and isinstance(fields[0], (SQL, Index)):
            cls._meta.indexes.append(fields[0])
        else:
            cls._meta.indexes.append(ModelIndex(cls, fields, **kwargs))


class ModelAlias(Node):
    """Provide a separate reference to a model in a query."""
    def __init__(self, model, alias=None):
        self.__dict__['model'] = model
        self.__dict__['alias'] = alias

    def __getattr__(self, attr):
        model_attr = getattr(self.model, attr)
        if isinstance(model_attr, Field):
            self.__dict__[attr] = FieldAlias.create(self, model_attr)
            return self.__dict__[attr]
        return model_attr

    def __setattr__(self, attr, value):
        raise AttributeError('Cannot set attributes on model aliases.')

    def get_field_aliases(self):
        return [getattr(self, n) for n in self.model._meta.sorted_field_names]

    def select(self, *selection):
        if not selection:
            selection = self.get_field_aliases()
        return ModelSelect(self, selection)

    def __call__(self, **kwargs):
        return self.model(**kwargs)

    def __sql__(self, ctx):
        if ctx.scope == SCOPE_VALUES:
            # Return the quoted table name.
            return ctx.sql(self.model)

        if self.alias:
            ctx.alias_manager[self] = self.alias

        if ctx.scope == SCOPE_SOURCE:
            # Define the table and its alias.
            return (ctx
                    .sql(self.model._meta.entity)
                    .literal(' AS ')
                    .sql(Entity(ctx.alias_manager[self])))
        else:
            # Refer to the table using the alias.
            return ctx.sql(Entity(ctx.alias_manager[self]))


class FieldAlias(Field):
    def __init__(self, source, field):
        self.source = source
        self.model = source.model
        self.field = field

    @classmethod
    def create(cls, source, field):
        class _FieldAlias(cls, type(field)):
            pass
        return _FieldAlias(source, field)

    def clone(self):
        return FieldAlias(self.source, self.field)

    def adapt(self, value): return self.field.adapt(value)
    def python_value(self, value): return self.field.python_value(value)
    def db_value(self, value): return self.field.db_value(value)
    def __getattr__(self, attr):
        return self.source if attr == 'model' else getattr(self.field, attr)

    def __sql__(self, ctx):
        return ctx.sql(Column(self.source, self.field.column_name))


def sort_models(models):
    models = set(models)
    seen = set()
    ordering = []
    def dfs(model):
        if model in models and model not in seen:
            seen.add(model)
            for rel_model in model._meta.refs.values():
                dfs(rel_model)
            if model._meta.depends_on:
                for dependency in model._meta.depends_on:
                    dfs(dependency)
            ordering.append(model)

    names = lambda m: (m._meta.name, m._meta.table_name)
    for m in sorted(models, key=names):
        dfs(m)
    return ordering


class _ModelQueryHelper(object):
    default_row_type = ROW.MODEL

    def __init__(self, *args, **kwargs):
        super(_ModelQueryHelper, self).__init__(*args, **kwargs)
        if not self._database:
            self._database = self.model._meta.database

    def _get_cursor_wrapper(self, cursor):
        row_type = self._row_type or self.default_row_type
        if row_type == ROW.MODEL:
            return self._get_model_cursor_wrapper(cursor)
        elif row_type == ROW.DICT:
            return ModelDictCursorWrapper(cursor, self.model, self._returning)
        elif row_type == ROW.TUPLE:
            return ModelTupleCursorWrapper(cursor, self.model, self._returning)
        elif row_type == ROW.NAMED_TUPLE:
            return ModelNamedTupleCursorWrapper(cursor, self.model,
                                                self._returning)
        elif row_type == ROW.CONSTRUCTOR:
            return ModelObjectCursorWrapper(cursor, self.model,
                                            self._returning, self._constructor)
        else:
            raise ValueError('Unrecognized row type: "%s".' % row_type)

    def _get_model_cursor_wrapper(self, cursor):
        return ModelObjectCursorWrapper(cursor, self.model, [], self.model)


class ModelRaw(_ModelQueryHelper, RawQuery):
    def __init__(self, model, sql, params, **kwargs):
        self.model = model
        self._returning = ()
        super(ModelRaw, self).__init__(sql=sql, params=params, **kwargs)

    def get(self):
        try:
            return self.execute()[0]
        except IndexError:
            sql, params = self.sql()
            raise self.model.DoesNotExist('%s instance matching query does '
                                          'not exist:\nSQL: %s\nParams: %s' %
                                          (self.model, sql, params))


class BaseModelSelect(_ModelQueryHelper):
    def __add__(self, rhs):
        return ModelCompoundSelectQuery(self.model, self, 'UNION ALL', rhs)

    def __or__(self, rhs):
        return ModelCompoundSelectQuery(self.model, self, 'UNION', rhs)

    def __and__(self, rhs):
        return ModelCompoundSelectQuery(self.model, self, 'INTERSECT', rhs)

    def __sub__(self, rhs):
        return ModelCompoundSelectQuery(self.model, self, 'EXCEPT', rhs)

    def __iter__(self):
        if not self._cursor_wrapper:
            self.execute()
        return iter(self._cursor_wrapper)

    def prefetch(self, *subqueries):
        return prefetch(self, *subqueries)

    def get(self):
        clone = self.paginate(1, 1)
        clone._cursor_wrapper = None
        try:
            return clone.execute()[0]
        except IndexError:
            sql, params = clone.sql()
            raise self.model.DoesNotExist('%s instance matching query does '
                                          'not exist:\nSQL: %s\nParams: %s' %
                                          (clone.model, sql, params))

    @Node.copy
    def group_by(self, *columns):
        grouping = []
        for column in columns:
            if is_model(column):
                grouping.extend(column._meta.sorted_fields)
            elif isinstance(column, Table):
                if not column._columns:
                    raise ValueError('Cannot pass a table to group_by() that '
                                     'does not have columns explicitly '
                                     'declared.')
                grouping.extend([getattr(column, col_name)
                                 for col_name in column._columns])
            else:
                grouping.append(column)
        self._group_by = grouping


class ModelCompoundSelectQuery(BaseModelSelect, CompoundSelectQuery):
    def __init__(self, model, *args, **kwargs):
        self.model = model
        super(ModelCompoundSelectQuery, self).__init__(*args, **kwargs)


class ModelSelect(BaseModelSelect, Select):
    def __init__(self, model, fields_or_models, is_default=False):
        self.model = self._join_ctx = model
        self._joins = {}
        self._is_default = is_default
        fields = []
        for fm in fields_or_models:
            if is_model(fm):
                fields.extend(fm._meta.sorted_fields)
            elif isinstance(fm, ModelAlias):
                fields.extend(fm.get_field_aliases())
            elif isinstance(fm, Table) and fm._columns:
                fields.extend([getattr(fm, col) for col in fm._columns])
            else:
                fields.append(fm)
        super(ModelSelect, self).__init__([model], fields)

    def select(self, *fields):
        if fields or not self._is_default:
            return super(ModelSelect, self).select(*fields)
        return self

    def switch(self, ctx=None):
        self._join_ctx = ctx
        return self

    @Node.copy
    def objects(self, constructor=None):
        self._row_type = ROW.CONSTRUCTOR
        self._constructor = self.model if constructor is None else constructor

    def _get_model(self, src):
        if is_model(src):
            return src, True
        elif isinstance(src, Table) and src._model:
            return src._model, False
        elif isinstance(src, ModelAlias):
            return src.model, False
        elif isinstance(src, ModelSelect):
            return src.model, False
        return None, False

    def _normalize_join(self, src, dest, on, attr):
        # Allow "on" expression to have an alias that determines the
        # destination attribute for the joined data.
        on_alias = isinstance(on, Alias)
        if on_alias:
            attr = on._alias
            on = on.alias()

        src_model, src_is_model = self._get_model(src)
        dest_model, dest_is_model = self._get_model(dest)
        if src_model and dest_model:
            self._join_ctx = dest
            constructor = dest_model

            if not (src_is_model and dest_is_model) and isinstance(on, Column):
                if on.source is src:
                    to_field = src_model._meta.columns[on.name]
                elif on.source is dest:
                    to_field = dest_model._meta.columns[on.name]
                else:
                    raise AttributeError('"on" clause Column %s does not '
                                         'belong to %s or %s.' %
                                         (on, src_model, dest_model))
                on = None
            elif isinstance(on, Field):
                to_field = on
                on = None
            else:
                to_field = None

            fk_field, is_backref = self._generate_on_clause(
                src_model, dest_model, to_field, on)

            if on is None:
                src_attr = 'name' if src_is_model else 'column_name'
                dest_attr = 'name' if dest_is_model else 'column_name'
                if is_backref:
                    lhs = getattr(dest, getattr(fk_field, dest_attr))
                    rhs = getattr(src, getattr(fk_field.rel_field, src_attr))
                else:
                    lhs = getattr(src, getattr(fk_field, src_attr))
                    rhs = getattr(dest, getattr(fk_field.rel_field, dest_attr))
                on = (lhs == rhs)

            if not attr:
                if fk_field is not None and not is_backref:
                    attr = fk_field.name
                else:
                    attr = dest_model._meta.name

        elif isinstance(dest, Source):
            constructor = dict
            attr = attr or dest._alias
            if not attr and isinstance(dest, Table):
                attr = attr or dest.__name__

        return (on, attr, constructor)

    @Node.copy
    def join(self, dest, join_type='INNER', on=None, src=None, attr=None):
        src = self._join_ctx if src is None else src

        on, attr, constructor = self._normalize_join(src, dest, on, attr)
        if attr:
            self._joins.setdefault(src, [])
            self._joins[src].append((dest, attr, constructor))

        return super(ModelSelect, self).join(dest, join_type, on)

    @Node.copy
    def join_from(self, src, dest, join_type='INNER', on=None, attr=None):
        return self.join(dest, join_type, on, src, attr)

    def _generate_on_clause(self, src, dest, to_field=None, on=None):
        meta = src._meta
        backref = fk_fields = False
        if dest in meta.model_refs:
            fk_fields = meta.model_refs[dest]
        elif dest in meta.model_backrefs:
            fk_fields = meta.model_backrefs[dest]
            backref = True

        if not fk_fields:
            if on is not None:
                return None, False
            raise ValueError('Unable to find foreign key between %s and %s. '
                             'Please specify an explicit join condition.' %
                             (src, dest))
        if to_field is not None:
            target = (to_field.field if isinstance(to_field, FieldAlias)
                      else to_field)
            fk_fields = [f for f in fk_fields if (
                         (f is target) or
                         (backref and f.rel_field is to_field))]

        if len(fk_fields) > 1:
            if on is None:
                raise ValueError('More than one foreign key between %s and %s.'
                                 ' Please specify which you are joining on.' %
                                 (src, dest))
            return None, False
        else:
            fk_field = fk_fields[0]

        return fk_field, backref

    def _get_model_cursor_wrapper(self, cursor):
        if len(self._from_list) == 1 and not self._joins:
            return ModelObjectCursorWrapper(cursor, self.model,
                                            self._returning, self.model)
        return ModelCursorWrapper(cursor, self.model, self._returning,
                                  self._from_list, self._joins)

    def ensure_join(self, lm, rm, on=None, **join_kwargs):
        join_ctx = self._join_ctx
        for dest, attr, constructor in self._joins.get(lm, []):
            if dest == rm:
                return self
        return self.switch(lm).join(rm, on=on, **join_kwargs).switch(join_ctx)

    def convert_dict_to_node(self, qdict):
        accum = []
        joins = []
        fks = (ForeignKeyField, BackrefAccessor)
        for key, value in sorted(qdict.items()):
            curr = self.model
            if '__' in key and key.rsplit('__', 1)[1] in DJANGO_MAP:
                key, op = key.rsplit('__', 1)
                op = DJANGO_MAP[op]
            elif value is None:
                op = OP.IS
            else:
                op = OP.EQ

            if '__' not in key:
                # Handle simplest case. This avoids joining over-eagerly when a
                # direct FK lookup is all that is required.
                model_attr = getattr(curr, key)
            else:
                for piece in key.split('__'):
                    for dest, attr, _ in self._joins.get(curr, ()):
                        if attr == piece or (isinstance(dest, ModelAlias) and
                                             dest.alias == piece):
                            curr = dest
                            break
                    else:
                        model_attr = getattr(curr, piece)
                        if value is not None and isinstance(model_attr, fks):
                            curr = model_attr.rel_model
                            joins.append(model_attr)
            accum.append(Expression(model_attr, op, value))
        return accum, joins

    def filter(self, *args, **kwargs):
        # normalize args and kwargs into a new expression
        dq_node = ColumnBase()
        if args:
            dq_node &= reduce(operator.and_, [a.clone() for a in args])
        if kwargs:
            dq_node &= DQ(**kwargs)

        # dq_node should now be an Expression, lhs = Node(), rhs = ...
        q = collections.deque([dq_node])
        dq_joins = set()
        while q:
            curr = q.popleft()
            if not isinstance(curr, Expression):
                continue
            for side, piece in (('lhs', curr.lhs), ('rhs', curr.rhs)):
                if isinstance(piece, DQ):
                    query, joins = self.convert_dict_to_node(piece.query)
                    dq_joins.update(joins)
                    expression = reduce(operator.and_, query)
                    # Apply values from the DQ object.
                    if piece._negated:
                        expression = Negated(expression)
                    #expression._alias = piece._alias
                    setattr(curr, side, expression)
                else:
                    q.append(piece)

        dq_node = dq_node.rhs

        query = self.clone()
        for field in dq_joins:
            if isinstance(field, ForeignKeyField):
                lm, rm = field.model, field.rel_model
                field_obj = field
            elif isinstance(field, BackrefAccessor):
                lm, rm = field.model, field.rel_model
                field_obj = field.field
            query = query.ensure_join(lm, rm, field_obj)
        return query.where(dq_node)

    def __sql_selection__(self, ctx, is_subquery=False):
        if self._is_default and is_subquery and len(self._returning) > 1 and \
           self.model._meta.primary_key is not False:
            return ctx.sql(self.model._meta.primary_key)

        return ctx.sql(CommaNodeList(self._returning))


class NoopModelSelect(ModelSelect):
    def __sql__(self, ctx):
        return self.model._meta.database.get_noop_select(ctx)

    def _get_cursor_wrapper(self, cursor):
        return CursorWrapper(cursor)


class _ModelWriteQueryHelper(_ModelQueryHelper):
    def __init__(self, model, *args, **kwargs):
        self.model = model
        super(_ModelWriteQueryHelper, self).__init__(model, *args, **kwargs)

    def _set_table_alias(self, ctx):
        table = self.model._meta.table
        ctx.alias_manager[table] = table.__name__


class ModelUpdate(_ModelWriteQueryHelper, Update):
    pass


class ModelInsert(_ModelWriteQueryHelper, Insert):
    def __init__(self, *args, **kwargs):
        super(ModelInsert, self).__init__(*args, **kwargs)
        if self._returning is None and self.model._meta.database is not None:
            if self.model._meta.database.returning_clause:
                self._returning = self.model._meta.get_primary_keys()
                self._row_type = ROW.TUPLE

    def get_default_data(self):
        return self.model._meta.defaults


class ModelDelete(_ModelWriteQueryHelper, Delete):
    pass


class ManyToManyQuery(ModelSelect):
    def __init__(self, instance, accessor, rel, *args, **kwargs):
        self._instance = instance
        self._accessor = accessor
        super(ManyToManyQuery, self).__init__(rel, (rel,), *args, **kwargs)

    def _id_list(self, model_or_id_list):
        if isinstance(model_or_id_list[0], Model):
            return [obj._pk for obj in model_or_id_list]
        return model_or_id_list

    def add(self, value, clear_existing=False):
        if clear_existing:
            self.clear()

        accessor = self._accessor
        if isinstance(value, SelectQuery):
            query = value.columns(
                SQL(str(self._instance._pk)),
                accessor.rel_model._meta.primary_key)
            accessor.through_model.insert_from(
                fields=[accessor.src_fk, accessor.dest_fk],
                query=query).execute()
        else:
            value = ensure_tuple(value)
            if not value:
                return
            inserts = [{
                accessor.src_fk.name: self._instance._pk,
                accessor.dest_fk.name: rel_id}
                for rel_id in self._id_list(value)]
            accessor.through_model.insert_many(inserts).execute()

    def remove(self, value):
        if isinstance(value, SelectQuery):
            subquery = value.columns(value.model._meta.primary_key)
            return (self._accessor.through_model
                    .delete()
                    .where(
                        (self._accessor.dest_fk << subquery) &
                        (self._accessor.src_fk == self._instance._pk))
                    .execute())
        else:
            value = ensure_tuple(value)
            if not value:
                return
            return (self._accessor.through_model
                    .delete()
                    .where(
                        (self._accessor.dest_fk << self._id_list(value)) &
                        (self._accessor.src_fk == self._instance._pk))
                    .execute())

    def clear(self):
        return (self._accessor.through_model
                .delete()
                .where(self._accessor.src_fk == self._instance)
                .execute())


class BaseModelCursorWrapper(DictCursorWrapper):
    def __init__(self, cursor, model, columns):
        super(BaseModelCursorWrapper, self).__init__(cursor)
        self.model = model
        self.select = columns or []

    def _initialize_columns(self):
        combined = self.model._meta.combined
        table = self.model._meta.table
        description = self.cursor.description

        self.ncols = len(self.cursor.description)
        self.columns = []
        self.converters = converters = [None] * self.ncols
        self.fields = fields = [None] * self.ncols

        for idx, description_item in enumerate(description):
            column = description_item[0]
            dot_index = column.find('.')
            if dot_index != -1:
                column = column[dot_index + 1:]

            self.columns.append(column)
            try:
                raw_node = self.select[idx]
            except IndexError:
                continue
            else:
                node = raw_node.unwrap()

            # Heuristics used to attempt to get the field associated with a
            # given SELECT column, so that we can accurately convert the value
            # returned by the database-cursor into a Python object.
            if isinstance(node, Field) and raw_node._coerce:
                converters[idx] = node.python_value
                fields[idx] = node
                if column == node.name or column == node.column_name:
                    self.columns[idx] = node.name
            elif column in combined:
                if not (isinstance(node, Function) and not node._coerce):
                    # Unlikely, but if a function was aliased to a column,
                    # don't use that column's converter if coerce is False.
                    converters[idx] = combined[column].python_value
                if isinstance(node, Column) and node.source == table:
                    fields[idx] = combined[column]
            elif (isinstance(node, Function) and node.arguments and
                  node._coerce):
                # Try to special-case functions calling fields.
                first = node.arguments[0]
                if isinstance(first, Node):
                    first = first.unwrap()

                if isinstance(first, Field):
                    converters[idx] = first.python_value
                elif isinstance(first, Entity):
                    path = first._path[-1]
                    field = combined.get(path)
                    if field is not None:
                        converters[idx] = field.python_value

    initialize = _initialize_columns

    def process_row(self, row):
        raise NotImplementedError


class ModelDictCursorWrapper(BaseModelCursorWrapper):
    def process_row(self, row):
        result = {}
        columns, converters = self.columns, self.converters
        fields = self.fields

        for i in range(self.ncols):
            attr = columns[i]
            if converters[i] is not None:
                result[attr] = converters[i](row[i])
            else:
                result[attr] = row[i]

        return result


class ModelTupleCursorWrapper(ModelDictCursorWrapper):
    constructor = tuple

    def process_row(self, row):
        columns, converters = self.columns, self.converters
        return self.constructor([
            (converters[i](row[i]) if converters[i] is not None else row[i])
            for i in range(self.ncols)])


class ModelNamedTupleCursorWrapper(ModelTupleCursorWrapper):
    def initialize(self):
        self._initialize_columns()
        attributes = []
        for i in range(self.ncols):
            attributes.append(self.columns[i])
        self.tuple_class = collections.namedtuple('Row', attributes)
        self.constructor = lambda row: self.tuple_class(*row)


class ModelObjectCursorWrapper(ModelDictCursorWrapper):
    def __init__(self, cursor, model, select, constructor):
        self.constructor = constructor
        self.is_model = is_model(constructor)
        super(ModelObjectCursorWrapper, self).__init__(cursor, model, select)

    def process_row(self, row):
        data = super(ModelObjectCursorWrapper, self).process_row(row)
        if self.is_model:
            # Clear out any dirty fields before returning to the user.
            obj = self.constructor(__no_default__=1, **data)
            obj._dirty.clear()
            return obj
        else:
            return self.constructor(**data)


class ModelCursorWrapper(BaseModelCursorWrapper):
    def __init__(self, cursor, model, select, from_list, joins):
        super(ModelCursorWrapper, self).__init__(cursor, model, select)
        self.from_list = from_list
        self.joins = joins

    def initialize(self):
        self._initialize_columns()
        selected_src = set([field.model for field in self.fields
                            if field is not None])
        select, columns = self.select, self.columns

        self.key_to_constructor = {self.model: self.model}
        self.src_is_dest = {}
        self.src_to_dest = []
        accum = collections.deque(self.from_list)
        dests = set()
        while accum:
            curr = accum.popleft()
            if isinstance(curr, Join):
                accum.append(curr.lhs)
                accum.append(curr.rhs)
                continue

            if curr not in self.joins:
                continue

            for key, attr, constructor in self.joins[curr]:
                if key not in self.key_to_constructor:
                    self.key_to_constructor[key] = constructor
                    self.src_to_dest.append((curr, attr, key,
                                             isinstance(curr, dict)))
                    dests.add(key)
                    accum.append(key)


        for src, _, dest, _ in self.src_to_dest:
            self.src_is_dest[src] = src in dests and (dest in selected_src
                                                      or src in selected_src)

        self.column_keys = []
        for idx, node in enumerate(select):
            key = self.model
            field = self.fields[idx]
            if field is not None:
                if isinstance(field, FieldAlias):
                    key = field.source
                else:
                    key = field.model
            else:
                if isinstance(node, Node):
                    node = node.unwrap()
                if isinstance(node, Column):
                    key = node.source

            self.column_keys.append(key)

    def process_row(self, row):
        objects = {}
        object_list = []
        for key, constructor in self.key_to_constructor.items():
            objects[key] = constructor(__no_default__=True)
            object_list.append(objects[key])

        set_keys = set()
        for idx, key in enumerate(self.column_keys):
            instance = objects[key]
            if self.fields[idx] is not None:
                column = self.fields[idx].name
            else:
                column = self.columns[idx]
            value = row[idx]
            if value is not None:
                set_keys.add(key)
            if self.converters[idx]:
                value = self.converters[idx](value)

            if isinstance(instance, dict):
                instance[column] = value
            else:
                setattr(instance, column, value)

        # Need to do some analysis on the joins before this.
        for (src, attr, dest, is_dict) in self.src_to_dest:
            instance = objects[src]
            try:
                joined_instance = objects[dest]
            except KeyError:
                continue

            # If no fields were set on the destination instance then do not
            # assign an "empty" instance.
            if instance is None or dest is None or \
               (dest not in set_keys and not self.src_is_dest.get(dest)):
                continue

            if is_dict:
                instance[attr] = joined_instance
            else:
                setattr(instance, attr, joined_instance)

        # When instantiating models from a cursor, we clear the dirty fields.
        for instance in object_list:
            if isinstance(instance, Model):
                instance._dirty.clear()

        return objects[self.model]


class PrefetchQuery(collections.namedtuple('_PrefetchQuery', (
    'query', 'fields', 'is_backref', 'rel_models', 'field_to_name', 'model'))):
    def __new__(cls, query, fields=None, is_backref=None, rel_models=None,
                field_to_name=None, model=None):
        if fields:
            if is_backref:
                rel_models = [field.model for field in fields]
                foreign_key_attrs = [field.rel_field.name for field in fields]
            else:
                rel_models = [field.rel_model for field in fields]
                foreign_key_attrs = [field.name for field in fields]
            field_to_name = list(zip(fields, foreign_key_attrs))
        model = query.model
        return super(PrefetchQuery, cls).__new__(
            cls, query, fields, is_backref, rel_models, field_to_name, model)

    def populate_instance(self, instance, id_map):
        if self.is_backref:
            for field in self.fields:
                identifier = instance.__data__[field.name]
                key = (field, identifier)
                if key in id_map:
                    setattr(instance, field.name, id_map[key])
        else:
            for field, attname in self.field_to_name:
                identifier = instance.__data__[field.rel_field.name]
                key = (field, identifier)
                rel_instances = id_map.get(key, [])
                for inst in rel_instances:
                    setattr(inst, attname, instance)
                setattr(instance, field.backref, rel_instances)

    def store_instance(self, instance, id_map):
        for field, attname in self.field_to_name:
            identity = field.rel_field.python_value(instance.__data__[attname])
            key = (field, identity)
            if self.is_backref:
                id_map[key] = instance
            else:
                id_map.setdefault(key, [])
                id_map[key].append(instance)


def prefetch_add_subquery(sq, subqueries):
    fixed_queries = [PrefetchQuery(sq)]
    for i, subquery in enumerate(subqueries):
        if isinstance(subquery, tuple):
            subquery, target_model = subquery
        else:
            target_model = None
        if not isinstance(subquery, Query) and is_model(subquery) or \
           isinstance(subquery, ModelAlias):
            subquery = subquery.select()
        subquery_model = subquery.model
        fks = backrefs = None
        for j in reversed(range(i + 1)):
            fixed = fixed_queries[j]
            last_query = fixed.query
            last_model = fixed.model
            rels = subquery_model._meta.model_refs.get(last_model, [])
            if rels:
                fks = [getattr(subquery_model, fk.name) for fk in rels]
                pks = [getattr(last_model, fk.rel_field.name) for fk in rels]
            else:
                backrefs = last_model._meta.model_refs.get(subquery_model, [])
            if (fks or backrefs) and ((target_model is last_model) or
                                      (target_model is None)):
                break

        if not fks and not backrefs:
            tgt_err = ' using %s' % target_model if target_model else ''
            raise AttributeError('Error: unable to find foreign key for '
                                 'query: %s%s' % (subquery, tgt_err))

        if fks:
            expr = reduce(operator.or_, [
                (fk << last_query.select(pk))
                for (fk, pk) in zip(fks, pks)])
            subquery = subquery.where(expr)
            fixed_queries.append(PrefetchQuery(subquery, fks, False))
        elif backrefs:
            expr = reduce(operator.or_, [
                (backref.rel_field << last_query.select(backref))
                for backref in backrefs])
            subquery = subquery.where(expr)
            fixed_queries.append(PrefetchQuery(subquery, backrefs, True))

    return fixed_queries


def prefetch(sq, *subqueries):
    if not subqueries:
        return sq

    fixed_queries = prefetch_add_subquery(sq, subqueries)
    deps = {}
    rel_map = {}
    for pq in reversed(fixed_queries):
        query_model = pq.model
        if pq.fields:
            for rel_model in pq.rel_models:
                rel_map.setdefault(rel_model, [])
                rel_map[rel_model].append(pq)

        deps[query_model] = {}
        id_map = deps[query_model]
        has_relations = bool(rel_map.get(query_model))

        for instance in pq.query:
            if pq.fields:
                pq.store_instance(instance, id_map)
            if has_relations:
                for rel in rel_map[query_model]:
                    rel.populate_instance(instance, deps[rel.model])

    return pq.query