How to define custom objects as query parameters in FastAPI (using PydanticV2 annotations)?

[DSamuylov]

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Example Code

import bson
from typing import Annotated

from pydantic import (
    field_validator,
    PlainSerializer,
    WithJsonSchema,
    BeforeValidator,
    TypeAdapter,
)
from fastapi import FastAPI


ObjectId = Annotated[
    bson.ObjectId,
    BeforeValidator(lambda x: bson.ObjectId(x) if isinstance(x, str) else x),
    PlainSerializer(lambda x: f"{x}", return_type=str),
    WithJsonSchema({"type": "string"}, mode="validation"),
    WithJsonSchema({"type": "string"}, mode="serialization"),
]

# Casting from str to ObjectId - WORKS
ta = TypeAdapter(
    ObjectId,
    config=dict(arbitrary_types_allowed=True),
)
ta.validate_python("5f7f9f0c2a0e0b0001d5b3d0")
ta.json_schema()

# Use ObjectId as a query parameter in FastAPI - FAILS:
app = FastAPI()

@app.get("/test")
def test(id: ObjectId) -> bool:
    return True

Description

I am trying to figure out how to update my FastAPI app after from Pydantic v1 to Pydantic v2.

Before I had code that worked perfectly fine:

class ObjectId(bson.ObjectId):
    @classmethod
    def __get_validators__(cls):
        yield cls.validate

    @classmethod
    def validate(cls, value):
        if isinstance(value, str):
            try:
                return bson.ObjectId(value)
            except bson.errors.InvalidId:
                raise ValueError("Invalid ObjectId")
        elif isinstance(value, bson.ObjectId):
            return value
        else:
            raise TypeError("ObjectId required")


app = FastAPI()

@app.get("/test")
def test(id: ObjectId) -> bool:
    return True

With the code that I provided above, I get the following error message:

fastapi.exceptions.FastAPIError: Invalid args for response field! Hint: check that <class 'bson.objectid.ObjectId'> is a valid Pydantic field type. If you are using a return type annotation that is not a valid Pydantic field (e.g. Union[Response, dict, None]) you can disable generating the response model from the type annotation with the path operation decorator parameter response_model=None. Read more: https://fastapi.tiangolo.com/tutorial/response-model/

I am a bit confused, because it seems that the problem is not with a response model (which is bool in the example), but with a custom type in query parameter.

I also checked other suggested approaches, for example:

• What is the new way to declare Mongo ObjectId with PyDantic v2.0^?
• Using bson.ObjectId in Pydantic v2

There, it is suggested an alternative approach to introduce a custom annotation type, however, it still does not work for me.

I also did not find any relevant in the FastAPI documentation about query parameters and a list of Extra Data Types.

I would highly appreciate if you could help to find a possible solution.

Operating System

macOS

Operating System Details

No response

FastAPI Version

0.101.1

Pydantic Version

2.1.1

Python Version

Python 3.11.4

Additional Context

No response

[romabozhanovgithub]

Hey, it seems like there is a problem. You can try this, but probably it is not, what you are looking for:

import bson
from typing import Annotated

from pydantic import (
    PlainSerializer,
    WithJsonSchema,
    BeforeValidator,
)
from fastapi import FastAPI


ObjectId = Annotated[
    str,
    BeforeValidator(lambda x: bson.ObjectId(x) if isinstance(x, str) else x),
    PlainSerializer(lambda x: f"{x}", return_type=str),
    WithJsonSchema({"type": "string"}, mode="validation"),
    WithJsonSchema({"type": "string"}, mode="serialization"),
]

app = FastAPI()

@app.get("/test", response_model=None)
def test(id: ObjectId) -> bool:
    return True

[DSamuylov]

Hi @romabozhanovgithub, thank you a lot for your comment. Yes, unfortunately this solution does not work because Annotated should not change the type, and all code interpreters would assume that ObjectId here is an instance of str.

Hopefully @tiangolo would have a moment to confirm if it is a bug or misunderstanding about how to use a new version of Pydantic with FastAPI.

[UlfurOrn]

I was looking for an answer to the same question and found a similar discussion here: #10131

It in turn gave me a solution that worked for me. Hopefully it also works in your case:

ObjectId = Annotated[
    bson.ObjectId,
    BeforeValidator(lambda x: bson.ObjectId(x) if isinstance(x, str) else x),
    PlainSerializer(lambda x: f"{x}", return_type=str),
    WithJsonSchema({"type": "string"}, mode="validation"),
    WithJsonSchema({"type": "string"}, mode="serialization"),
    Query(),
]

or

@app.get("/test", response_model=None)
def test(id: Annotated[ObjectId, Query()]) -> bool:
    return True

I think the docs may benefit from adding an example for this usecase 🤔

[ThoundsN]

This exactly solved my issue!

[DSamuylov]

Hi @UlfurOrn, thank you a lot for your suggestion, but for me the code still fails with:

fastapi.exceptions.FastAPIError: Invalid args for response field! Hint: check that <class 'bson.objectid.ObjectId'> is a valid Pydantic field type. If you are using a return type annotation that is not a valid Pydantic field (e.g. Union[Response, dict, None]) you can disable generating the response model from the type annotation with the path operation decorator parameter response_model=None. Read more: https://fastapi.tiangolo.com/tutorial/response-model/

Does it run for you?

[DSamuylov]

Actually I am not sure that the code from #10131 could be adjusted to my case, because MyNumber is a child of int which is a supported data type.

[JavierSanchezCastro]

Check this: https://devpress.csdn.net/mongodb/6303c11dc67703293080a14d.html

[DSamuylov]

Hi @JavierSanchezCastro, thank you for the link, but that discussion is about how to do it with Pydantic V1 and this is what I had before trying to update to Pydantic V2. Unfortunately, it does not answer the question.

[DSamuylov]

Anybody has an idea on how to fix it? It is still a problem...

[omi-donlimit]

Same here. Did you find any workaround in the meantime?

[Kludex]

• Created How to define custom objects as query parameters in FastAPI (using PydanticV2 annotations)? #10259.