Fastapi: [QUESTION] Response model with pydantic's arbitrary_types_allowed not working

[datasatanic]

I use pydantic model with custom type. Set in config arbitrary_types_allowed=True.
Without setting a parameter "response_model" it's work. But when I set this model in param:

fastapi.exceptions.FastAPIError: Invalid args for response field! Hint: check that <class 'Foo'> is a valid pydantic field type 

Pydantic class definition example:

import Foo

class Model(BaseModel):
    id: str
    custom: Optional[Foo] = Field(None)
    status: str = None
    fail_message: Optional[str] = None
    params: Dict[str, Any] = None

    @validator('custom')
    def deserialize_model(cls, v):
        return Foo.loads(v)

    class Config:
        arbitrary_types_allowed = True
        json_encoders = {Foo: lambda v: v.dumps()}

[andreipopovici]

I see the same issue with an Arrow type returned from the SQLAlchemy model. arbitrary_types_allowed is not working when using the pydantic model in the response_model.

[tiangolo]

Please create a small self-contained example with a minimal FastAPI app that shows the error and what you would expect to happen, that way I can understand what's happening and what you want to achieve.

[github-actions[bot]]

Assuming the original issue was solved, it will be automatically closed now. But feel free to add more comments or create new issues.

[bcb]

@tiangolo I think the config is getting lost in the create_cloned_field function.

new_field = create_response_field(name=field.name, type_=use_type)

Shouldn't it also pass model_config?

[bcb]

I ended up using a custom Pydantic field validator, instead of using arbitrary_types_allowed:

class AssociationList(_AssociationList):
    @classmethod
    def __get_validators__(cls):
        yield cls.validate

    @classmethod
    def validate(cls, v):
        if not isinstance(v, _AssociationList):
            raise TypeError("AssociationList required")
        return [UserOut.from_orm(u) for u in v]


class TeamOut(TeamBase):
    members: AssociationList
    ...

[fubupc]

Same problem here:

from pydantic import BaseModel
from fastapi import FastAPI


class Pair:
    def __init__(self, x: int, y: int):
        self.x: int = x
        self.y: int = y


class M(BaseModel):
    pair: Pair
    other: int

    class Config:
        arbitrary_types_allowed = True


app = FastAPI()


@app.get("/m/", response_model=M)
def just_m():
    return M(pair=Pair(x=5, y=7), other=8)

will report error:

fastapi.exceptions.FastAPIError: Invalid args for response field! Hint: check that <class 'test.Pair'> is a valid pydantic field type