[Python] AttributeError: get_motor_collection when setting fetch_links=True

I'm using Beanie with FastAPI. For example, my Beanie models look like this:

class Project(Document):
id: PydanticObjectId = Field(default_factory=PydanticObjectId)
items: list[Link[ItemDB]] | None = None

class ItemDB(Document):
creator: Indexed(str)


When getting a Project by ID, I also want to prefetch its related items. So, my endpoint looks like this:

@router.get('/{project_id}', status_code=status.HTTP_200_OK, summary='Get a project',
response_model_exclude_none=True)
async def get_project(project_id: str) -> Project:
project = await Project.get(project_id, fetch_links=True)
if not project:
raise HTTPException(status_code=status.HTTP_404_NOT_FOUND)
return project


But I got the error:

ERROR: Exception in ASGI application
Traceback (most recent call last):
File "my-project/.venv/lib/python3.12/site-packages/uvicorn/protocols/http/httptools_impl.py", line 419, in run_asgi
result = await app( # type: ignore[func-returns-value]
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "my-project/.venv/lib/python3.12/site-packages/uvicorn/middleware/proxy_headers.py", line 84, in __call__
return await self.app(scope, receive, send)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "my-project/.venv/lib/python3.12/site-packages/fastapi/applications.py", line 1054, in __call__
await super().__call__(scope, receive, send)
File "my-project/.venv/lib/python3.12/site-packages/starlette/applications.py", line 113, in __call__
await self.middleware_stack(scope, receive, send)
File "my-project/.venv/lib/python3.12/site-packages/starlette/middleware/errors.py", line 187, in __call__
raise exc
File "my-project/.venv/lib/python3.12/site-packages/starlette/middleware/errors.py", line 165, in __call__
await self.app(scope, receive, _send)
File "my-project/.venv/lib/python3.12/site-packages/starlette/middleware/exceptions.py", line 62, in __call__
await wrap_app_handling_exceptions(self.app, conn)(scope, receive, send)
File "my-project/.venv/lib/python3.12/site-packages/starlette/_exception_handler.py", line 62, in wrapped_app
raise exc
File "my-project/.venv/lib/python3.12/site-packages/starlette/_exception_handler.py", line 51, in wrapped_app
await app(scope, receive, sender)
File "my-project/.venv/lib/python3.12/site-packages/starlette/routing.py", line 715, in __call__
await self.middleware_stack(scope, receive, send)
File "my-project/.venv/lib/python3.12/site-packages/starlette/routing.py", line 735, in app
await route.handle(scope, receive, send)
File "my-project/.venv/lib/python3.12/site-packages/starlette/routing.py", line 288, in handle
await self.app(scope, receive, send)
File "my-project/.venv/lib/python3.12/site-packages/starlette/routing.py", line 76, in app
await wrap_app_handling_exceptions(app, request)(scope, receive, send)
File "my-project/.venv/lib/python3.12/site-packages/starlette/_exception_handler.py", line 62, in wrapped_app
raise exc
File "my-project/.venv/lib/python3.12/site-packages/starlette/_exception_handler.py", line 51, in wrapped_app
await app(scope, receive, sender)
File "my-project/.venv/lib/python3.12/site-packages/starlette/routing.py", line 73, in app
response = await f(request)
^^^^^^^^^^^^^^^^
File "my-project/.venv/lib/python3.12/site-packages/fastapi/routing.py", line 301, in app
raw_response = await run_endpoint_function(
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "my-project/.venv/lib/python3.12/site-packages/fastapi/routing.py", line 212, in run_endpoint_function
return await dependant.call(**values)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "my-project/app/routers/project.py", line 46, in get_project
project = await Project.get(project_id, fetch_links=True)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "my-project/.venv/lib/python3.12/site-packages/beanie/odm/documents.py", line 276, in get
return await cls.find_one(
^^^^^^^^^^^^^^^^^^^
File "my-project/.venv/lib/python3.12/site-packages/beanie/odm/queries/find.py", line 1024, in __await__
document = yield from self._find_one().__await__() # type: ignore
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "my-project/.venv/lib/python3.12/site-packages/beanie/odm/queries/find.py", line 982, in _find_one
return await self.document_model.find_many(
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "my-project/.venv/lib/python3.12/site-packages/beanie/odm/queries/find.py", line 689, in first_or_none
res = await self.limit(1).to_list()
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "my-project/.venv/lib/python3.12/site-packages/beanie/odm/queries/cursor.py", line 67, in to_list
cursor = self.motor_cursor
^^^^^^^^^^^^^^^^^
File "my-project/.venv/lib/python3.12/site-packages/beanie/odm/queries/find.py", line 661, in motor_cursor
self.build_aggregation_pipeline()
File "my-project/.venv/lib/python3.12/site-packages/beanie/odm/queries/find.py", line 610, in build_aggregation_pipeline
construct_lookup_queries(
File "my-project/.venv/lib/python3.12/site-packages/beanie/odm/utils/find.py", line 29, in construct_lookup_queries
construct_query(
File "my-project/.venv/lib/python3.12/site-packages/beanie/odm/utils/find.py", line 271, in construct_query
"from": link_info.document_class.get_motor_collection().name, # type: ignore
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "my-project/.venv/lib/python3.12/site-packages/pydantic/_internal/_model_construction.py", line 242, in __getattr__
raise AttributeError(item)
AttributeError: get_motor_collection


If I remove fetch_links=True and later on call

await project.fetch_all_links()


then it works fine.

I'm sure that the database was initialized properly because everything else works normally, only this fetch_links has problem. Just to be sure, here is how the database was initialized:

async def init_db(db_url: str):
client = AsyncIOMotorClient(host=db_url, tz_aware=True)
await init_beanie(
database=client.get_default_database(default='myDB'),
document_models=[Project, ItemDB],
allow_index_dropping=True
)


This function is then called in the startup part of FastAPI:

@asynccontextmanager
async def lifespan(app: FastAPI):
await init_db(settings.db_url)
yield


app = FastAPI(
lifespan=lifespan
)


So, did I miss something? Or it's a bug of Beanie? I'm using FastAPI 0.115.2 and Beanie 1.27.0.

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