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Example Code

# this example is copied from the Pydantic docs with updates to work with FastAPI
# docs: https://pydantic.dev/docs/validation/latest/concepts/serialization/#polymorphic-serialization

from fastapi import FastAPI, Response
from pydantic import BaseModel, ConfigDict

class User(BaseModel):
    name: str


class UserLogin(User):
    password: str


class OuterModel(BaseModel):
    # would love if FastAPI could read this!
    model_config = ConfigDict(polymorphic_serialization=True)

    user: User


app = FastAPI()


@app.get("/")
def current():
    model = OuterModel(
        user=UserLogin(name='pydantic', password='password'),
    )
    return model


@app.get("/expected")
def expected():
    model = OuterModel(
        user=UserLogin(name='pydantic', password='password'),
    )
    content = model.model_dump_json(polymorphic_serialization=True)
    return Response(media_type="application/json", content=content)

Description

• Pydantic 2.13+ uses polymorphic_serialization=True to serialize a model with its actual type vs declared type
  • PR: Add polymorphic_serialization option pydantic/pydantic#12518
  • Release notes: https://pydantic.dev/articles/pydantic-v2-13-release
• When serializing a response it'd be great to either:
  • Be able to declare in the route to pass this flag when serializing (e.g., @app.get(..., polymorphic_serialization=True)), or...
  • For FastAPI to peek into the model's config to see if the flag is there, and if so apply it (this would be awesome!)

Operating System

Linux, macOS

Operating System Details

No response

FastAPI Version

0.135.3

Pydantic Version

2.13.4

Python Version

3.11.14

Additional Context

No response