[BUG] Issue with List[Link[Type]] when fetch_all_links is called

[space55]

Describe the bug
As discussed in the Discord, when using List[Link[Type]], the following error is received:

  File "/Users/space/.local/share/virtualenvs/api-server-master-haddR1V9/lib/python3.10/site-packages/beanie/odm/documents.py", line 1060, in fetch_all_links
    await asyncio.gather(*coros)
  File "/Users/space/.local/share/virtualenvs/api-server-master-haddR1V9/lib/python3.10/site-packages/beanie/odm/documents.py", line 1051, in fetch_link
    values = await Link.fetch_list(ref_obj, fetch_links=True)
  File "/Users/space/.local/share/virtualenvs/api-server-master-haddR1V9/lib/python3.10/site-packages/beanie/odm/fields.py", line 217, in fetch_list
    fetched_models = await model_class.find(  # type: ignore
AttributeError: 'NoneType' object has no attribute 'find'

To Reproduce

import asyncio
from typing import Optional, List

from motor.motor_asyncio import AsyncIOMotorClient

from beanie import Document, Indexed, init_beanie, Link


class Bar(Document):
    value: str


class Foo(Document):
    name: str
    bars: Optional[List[Link[Bar]]] = []


async def example():
    client = AsyncIOMotorClient("mongodb://admin:admin@localhost:27017")
    await init_beanie(database=client.db_name, document_models=[Foo, Bar])

    bar = Bar(value="World")
    await bar.save()

    foo = Foo(name="Hello", bars=[bar])
    await foo.save()

    await foo.fetch_all_links()


if __name__ == "__main__":
    asyncio.run(example())

Expected behavior
Not this error

Additional context
Running with Motor, and the Optional is potentially important. I always recreate this with a completely fresh database running in docker. Please let me know if you have any issues reproducing this, or any other questions!

[voelcker-itsc]

Encounterd the same problem for mandatory fields, if I used fetch_links=True in the find method.
A quick debug later, I guess the problem is in the fetch_list method if the data list is empty (optional field) or no link is a Link (already fetched). So model_class will never get set.

[roman-right]

This will be fixed in #669

[github-actions]

This issue is stale because it has been open 30 days with no activity.

[github-actions]

This issue was closed because it has been stalled for 14 days with no activity.