[Python] How to add objects/links to a set of links in beanie?

Assume that I have these Beanie Documents which are based, by the way, on Pydantic Models:

File name: models.py

from beanie import Document, Link

class A(Document):
first: int
second: str

class B(Document):
third: float
a_links: set[Link[A]] = {}


and I have this FastAPI route:

File name: main.py

from fastapi import FastAPI, HTTPException, status

from beanie import PydanticObjectId, Link

from .models import A, B

app = FastAPI()

@app.post('/b/{b_object_id}/add-link/{a_object_id}')
async def add_link(b_object_id: PydanticObjectId, a_object_id: PydanticObjectId):
b = await B.get(b_object_id)
if not b:
raise HTTPException(status_code=status.HTTP_404_NOT_FOUND)
a = await A.get(a_object_id)
if not a:
raise HTTPException(status_code=status.HTTP_404_NOT_FOUND)
b.a_links.add(Link(a))
await b.save()
return b


I am talking about this line of code:

b.a_links.add(Link(a))


If I wrote it as it is, I will get this error: Parameter 'document_class' unfilled

Also, if I wrote it as:

b.a_links.add(Link(a, document_class=A))


I will get this error: Expected type 'DBRef', got 'A' instead

Finally, if I wrote it as:

b.a_links.add(Link(ref=a.id, document_class=A))


I will get this error: Expected type 'DBRef', got 'PydanticObjectId | None' instead

How to add it in a correct way?

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