Allow change class_id and use name settings in UnionDoc

[wonjoonSeol-WS]

Beanie is all sound and good until there is a need to connect to existing DB where beanie is/was not used.
For instance, one might have SNS -> SQS -> lambda (that uses any other client other than beanie) -> DocDB / Mongo.
In case like this, the existing data would not have beanie's internal data field such as '_class_id".

This makes switching to Beanie difficult as it requires full DB migration, especially if DB is used for event-sourcing.

This PR allows (1) custom internal data field name to be set and (2) custom name to be set in Union doc children classes.

Usage:

class Parent(UnionDoc):
    class Settings:
        class_id = "event_type" <-
        name = "eventsource"

class ChildCreated(Document):
    event_type: Literal["created"] = "created" <- event_type is used instead of _class_id 
    sent_at: datetime

    class Settings:
        name = "created" <- This was option was ignored before
        union_doc = Parent

class ChildDeleted(Document):
    event_type: Literal["created"] = "deleted" <- event_type is used instead of _class_id 
    sent_at: datetime

    class Settings:
        name = "deleted" <- This was option was ignored before
        union_doc = Parent

Example:

Schema

class Parent(UnionDoc):
    class Settings:
        class_id = "event_type"
        name = "collection"


class One(Document):
    test: int

    class Settings:
        name = "two"
        class_id = "event_type"
        union_doc = Parent
        
 class Two(Document): <- without union_doc
    test: int

    class Settings:
        name = "three"
        class_id = "event_type"

Test

In [1]: await db.One(test=1).insert()
Out[1]: One(id=ObjectId('63ae1890edab780d45df0b07'), revision_id=None, test=1)

pymongo
In [17]: list(c["db"]["collection"].find({'test': 1}))
Out[17]: [{'_id': ObjectId('63ae1890edab780d45df0b07'), 'event_type': 'two', 'test': 1}]

In [1]: await db.Two(test=3).insert()
Out[1]: Two(id=ObjectId('63ae19cd8e684a4cd769fb1f'), revision_id=None, test=3)

pymongo
In [23]: list(c["db"]["three"].find({'test': 3}))
Out[23]: [{'_id': ObjectId('63ae19cd8e684a4cd769fb1f'), 'test': 3}]

---

FYI, #206 this PR was not reviewed for a year so I closed it..

[wonjoonSeol-WS]

if cls.get_settings().name always returns original class name instead of None when name is not set, we wouldn't need this line.

[wonjoonSeol-WS]

Does it ever reach line 29 for UnionDoc? it didn't for my (small) dataset

[wonjoonSeol-WS]

        if class_name not in model._document_models:  # type: ignore
            raise DocWasNotRegisteredInUnionClass

This line seems too strict - especially in event source DB. One MSA can add a new event and boom we now have an error. if there is schema that beanie doesn't yet know perhaps gracefully skip?

[roman-right]

Hi @wonjoonSeol-WS ,
Sounds very reasonable. I'll check the PR soon this week

[github-actions]

This PR is stale because it has been open 45 days with no activity.

[roman-right]

Merged. Thank you very mush for the PR.
It will be published in the next release (beta or normal) today-tomorrow