[BUG] Calling `init_beanie` twice with different models results in object ID permanently set to `_id`

[sstraub-hmmc]

Describe the bug

When a custom base class is used which inherits from beanie.Document, the base class gets modified when init_beanie() is called. This leads to bugs later on if init_beanie() is called again with a different model that inherits from the same class.

Usually I try to avoid calling init_beanie() multiple times, but I was writing a tutorial in a Python notebook and there I ran into this issue...

To Reproduce

from beanie import Document, init_beanie
from pymongo import AsyncMongoClient


async def test_inheritance_bug():
    client = AsyncMongoClient("mongodb://root:secret@localhost:27017")
    database = client.get_database("test")

    class A(Document):
        a: str | None = None

    await init_beanie(database=database, document_models=[A])
    a = A(a="foo")
    assert a.id is None

    class B(Document):
        b: str | None = None

    await init_beanie(database=database, document_models=[B])
    b = B(b="bar")
    assert b.id is None

    class MyDocument(Document):
        pass

    class A2(MyDocument):
        a: str | None = None

    await init_beanie(database=database, document_models=[A2])
    a2 = A2(a="foo")
    assert a2.id is None

    class B2(MyDocument):
        b: str | None = None

    await init_beanie(database=database, document_models=[B2])
    b2 = B2(b="bar")
    # THIS FAILS: B2(id='_id', revision_id='revision_id', b='bar')
    assert b2.id is None

Expected behavior

It should be possible to call init_beanie() multiple times during the runtime of an application without things breaking (even if it's not best practice). In this case, things are breaking

Additional context

When init_beanie is called with MyDocument for the first time, two new fields are assigned to the MyDocument type: id: ExpressionField = '_id' and revision_id: ExpressionField = 'revision_id'. This causes subsequent failures, since now every new instance of the model has '_id' as object id, which is invalid.

The issue does not come up if you declare B2 before you call init_beanie, even if you don't add B2 to the document_models. In any case, this is not a viable workaround for me, since I need to be able to declare new models later on in my jupyter notebook.