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Closes #6286

Respect the definite assignment assertion operator in the entity
'Article'. Without assigning the previously missing class variables,
TypeScript complains about the same.

Closes mikro-orm/guide#5

### The Problem

Related issue:
[#6265](#6265). Before the
changes, populating a `mapToPk` relation did not extract just the key;
instead, the entire entity object was assigned to the property.

### The Solution

I have added tests for both simple and composite primary keys. While
adding tests for the `select-in` loading strategy, I noticed that the
second query (for loading the primary key entity) is not executed. This
makes sense because the keys are already selected.

This led me to reconsider whether we should use a join for the `joined`
strategy, as the keys are already present on the owning entity.

**Select-in strategy:**

```sql
select `t0`.* 
from `team` as `t0` 
where `t0`.`id` = 'team1' 
limit 1
```
**Joined strategy:**

```sql
select `t0`.*, `c1`.`id` as `c1__id`
from `team` as `t0`
-- this join is redundant
left join `order` as `c1` on `t0`.`current_order_id` = `c1`.`id`
where `t0`.`id` = 'team1'
limit 1
```

EDIT: I have also added a test for the inverse side of a 1:1 relation
using `mapToPk` to verify that it works as expected. Another redundant
populate query was being made to the `Team` entity, which I addressed as
well.

```sql
select `o0`.*, `o1`.`id` as `o1__id`
from `order` as `o0`
left join `team` as `o1` on `o0`.`id` = `o1`.`current_order_id`
where `o0`.`id` = 'order1'
limit 1
-- this populate query is redundant because we do not want the full entity
select `t0`.*
from `team` as `t0`
where `t0`.`current_order_id` in ('order1')
```

…and QueryBuilder result (#6307)

If you query entity already in identityMap, its serialization options
will not be modified from joins from the query builder. This concats the
populate options just like in `setSerializationOptions` so all fields
will be populated from the identity map and inserted into the serialized
result

…hen it is removed from a collection (#6265)

### The Problem

When using a `toMany` relation on an entity and also adding a `formula`
that references the same entity in the collection, removing the entity
from the collection causes the `formula` relation to keep referencing
it, resulting in the deleted entity being reinserted on flush.

### Description of the Solution

I updated `referencingProperties` to allow multiple references on the
same model, not just the first. However, the problem persisted because
even though `unsetIdentity` performs `delete
helper(referrer).__data[prop.name];`, it does nothing since `__data` is
an empty object. I added an explicit override of the reference with
`referrer[prop.name] = null;`.

I’m not sure what the correct way to reset this reference is (another
test is failing because of that), so advice would be appreciated.
Options include:
- Setting it to `null`  
- Setting it to `undefined`  
- Deleting the property with `delete referrer[prop.name]`

---------

Co-authored-by: Martin Adámek <banan23@gmail.com>

…t()`

Closes #6291

More info in dsherret/ts-morph#1593

Closes #6297

Closes #6308

Closes #6308