Nested objects/documents allow to map certain sections in the document indexed as nested allowing to query them as if they are separate docs joining with the parent owning doc.

Note, this feature is experimental and might require reindexing the data if using it.

One of the problems when indexing inner objects that occur several times in a doc is that "cross object" search match will occur, for example:

{
    "obj1" : [
        {
            "name" : "blue",
            "count" : 4
        },
        {
            "name" : "green",
            "count" : 6
        }
    ]
}

Searching for name set to blue and count higher than 5 will match the doc, because in the first element the name matches blue, and in the second element, count matches "higher than 5".

Nested Mapping

Nested mapping allow to map certain inner objects (usually multi instance ones), for example:

{
    "type1" : {
        "properties" : {
            "obj1" : {
                "type" : "nested"
            }
        }
    }
}

The above will cause all obj1 to be indexed as a nested doc. The mapping is similar in nature to setting type to object, except that its nested.

The nested object fields can also be automatically added to the immediate parent by setting include_in_parent to true, and also included in the root object by setting include_in_root to true.

Nested docs will also automatically use the root doc _all field.

Nested Queries

Nested queries allow to search within nested docs, resulting in the root parent doc (join), for example:

{
    "nested" : {
        "path" : "obj1",
        "score_mode" : "avg"
        "query" : {
            "bool" : {
                "must" : [
                    "text" : {"obj1.name" : "blue"},
                    "range" : {"obj1.count" : {"gt" : 5}}
                ]
            }
        }
    }
}

The query path points to the nested object path, and the query (or filter) includes the query that will run on the nested docs matching the direct path, and joining with the root parent docs.

The score_mode allows to set how inner children matching affects scoring of parent. It defaults to avg, but can be total, max and none.

Multi level nesting is automatically supported, and detected, resulting in an inner nested query to automatically match the relevant nesting level (and not root) if it exists within another nested query.

Internal Implementation

Internally, nested objects are indexed as additional documents, but, since they can be guaranteed to be indexed within the same "block", it allows for extremely fast joining with parent docs.

Those internal nested documents are automatically masked away when doing operations against the index (like searching with a match_all query), and they bubble out when using the nested query.

Left Overs

There are many things to still do to have it as a complete. The most important one is have a good story around faceting and nested docs (it will really enhance it as well, especially for key and value based facets).