I followed the doc to create a relationship, but it doesn't work

[tanguynicolas]

Hello,

I'm starting with Beanie. I create an API with FastAPI and MongoDB.

Here's the code, which I've simplified for you, that doesn't work. I've (I think) done exactly what the official documentation says. I don't understand why it can't work:

from typing import Optional, List

from fastapi import FastAPI
from motor.motor_asyncio import AsyncIOMotorClient
from beanie import init_beanie, Document, Link, BackLink
from pydantic import Field

##### MODELS #####

class Team(Document):
    name: str
    players: Optional[List[BackLink["Player"]]] = Field(json_schema_extra={"original_field": "team"})

    class Settings:
        name = "teams"

class Player(Document):
    name: str
    team: Link[Team]

    class Settings:
        name = "players"

##### DATABASE #####

async def init():
    # Create Motor client
    client = AsyncIOMotorClient("mongodb://manager:password@localhost:27017")

    # Init beanie with the document class
    await init_beanie(database=client["debug"], document_models=[Team, Player])

##### MAIN #####

app = FastAPI()

app.add_event_handler("startup", init)

@app.post("/")
async def create_team(team: Team):
    result = await team.insert()
    return result

In fact, uvicorn runs successfully, but when I launch the Swagger UI, I get an error. This is due to my relationship between Team and Player, because when I remove it, everything works (or when I remove team: Team from create_team(team: Team)).

Fetch error
Internal Server Error /openapi.json

INFO:     127.0.0.1:45658 - "GET /openapi.json HTTP/1.1" 500 Internal Server Error
ERROR:    Exception in ASGI application
Traceback (most recent call last):
  File "/home/user/project/.venv/lib/python3.11/site-packages/uvicorn/protocols/http/httptools_impl.py", line 411, in run_asgi
    result = await app(  # type: ignore[func-returns-value]
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/user/project/.venv/lib/python3.11/site-packages/uvicorn/middleware/proxy_headers.py", line 69, in __call__
    return await self.app(scope, receive, send)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

...

  File "/home/user/project/.venv/lib/python3.11/site-packages/pydantic/json_schema.py", line 1019, in function_plain_schema
    return self._function_schema(schema)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/user/project/.venv/lib/python3.11/site-packages/pydantic/json_schema.py", line 984, in _function_schema
    return self.handle_invalid_for_json_schema(
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/user/project/.venv/lib/python3.11/site-packages/pydantic/json_schema.py", line 2152, in handle_invalid_for_json_schema
    raise PydanticInvalidForJsonSchema(f'Cannot generate a JsonSchema for {error_info}')

pydantic.errors.PydanticInvalidForJsonSchema: Cannot generate a JsonSchema for core_schema.PlainValidatorFunctionSchema ({'type': 'with-info', 'function': <function BackLink.build_validation.<locals>.validate at 0x7f470dce0040>})

My versions:
fastapi 0.111.0
pydantic 2.7.1
motor 3.4.0
beanie 1.26.0
uvicorn 0.29.0

I'm sure it's a simple mistake, but unfortunately I can't figure it out. A more experienced eye could help me.

Thank you

Bonus: if you see things to improve in my code, don't hesitate, it will be very instructive for me!

[hgalytoby]

#577 (comment)