FastAPI shows no attributes for models when a @model_serializer is used.

[TNick]

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Example Code

from typing import Any, Dict
from fastapi import FastAPI
from pydantic import BaseModel, Field, SerializationInfo, model_serializer


class SomeModel(BaseModel):
    lorem: str = Field(..., description="describe me")
    ipsum: int

    @model_serializer(when_used="always", mode="wrap")
    def clean_model(self, serializer: Any, info: SerializationInfo) -> Dict[str, Any]:
        data: Dict[str, Any] = serializer(self)
        return data

app = FastAPI()

@app.get("/another/{dataset}", response_model=SomeModel)
def get_another_dataset(dataset: str):
    raise NotImplementedError

Description

• run provided code
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Operating System

Linux, Windows, macOS, Other

Operating System Details

No response

FastAPI Version

0.104.1

Pydantic Version

2.5.1

Python Version

3.9

Additional Context

Uncovered while investigating this geojson-pydantic issue.

Edit: code sandbox available here.

[volbil]

I'm facing same issue. I think it can be narroved down to Pydantic's model with @model_serializer passed to FastAPI's response_model field. Models used for endpoint arguments do display attributes.

[DLukeNelson]

I ran into this recently, and I found a workaround. In my case, removing the Dict[str, Any] type hint for clean_model's return value fixed the issue for me.
I couldn't say why this works, I just happened to be trying to create an MVE, and left off the type hint. Surprisingly, it worked correctly.

[alex-pobeditel-2004]

@DLukeNelson you're my savior!
After move to Pydantic v2, all my models (that use common base model) lost OpenAPI response model schemas - now I see that happened because of serializer that I added because pydantic didn't serialize Path and AnyUrl objects correctly.

Without type annotation on serializer, everything works like a charm!

@tiangolo @Kludex I don't know if this is a FastAPI or Pydantic issue, but I hope that real problem solution will make it to release once :)

[eseglem]

The best workaround I have found is to not type hint the return on the serializer. Then it will use the same definition as it would without the serializer defined.

    def clean_model(self, serializer: Any, info: SerializationInfo):  # type: ignore [no-untyped-def]

From the FastAPI side of things this is because it calls .model_json_schema(mode="serialization") and typing the return of the serializer changes what pydantic returns for the serialization mode. I feel like what FastAPI is doing is correct and the root of the issue is on the pydantic side. I wrote up an issue on pydantic requesting a nicer way to handle this: pydantic/pydantic#8791 if anyone wants thumbs up it or chime in with thoughts.