Cannot use a pydantic `dataclass` nested in a `BaseModel` as a `response_model`

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Example Code

from fastapi import FastAPI
from pydantic import BaseModel
from pydantic.dataclasses import dataclass


app = FastAPI()


@dataclass
class Item:
    name: str


class Box(BaseModel):
    item: Item


@app.get("/item", response_model=Item)
async def item() -> Item:
    return Item(name="test")


@app.get("/box", response_model=Box)
async def box() -> Box:
    return Box(item=await item())

Description

The item endpoint works fine, but the box one fails with a ValidationError:

$ uvicorn main:app
INFO:     Started server process [13673]
INFO:     Waiting for application startup.
INFO:     Application startup complete.
INFO:     Uvicorn running on http://127.0.0.1:8000 (Press CTRL+C to quit)
INFO:     127.0.0.1:46020 - "GET /docs HTTP/1.1" 200 OK
INFO:     127.0.0.1:46020 - "GET /openapi.json HTTP/1.1" 200 OK
INFO:     127.0.0.1:46020 - "GET /item HTTP/1.1" 200 OK
INFO:     127.0.0.1:46020 - "GET /box HTTP/1.1" 500 Internal Server Error
ERROR:    Exception in ASGI application
Traceback (most recent call last):
  File "/home/guest/.local/share/virtualenvs/tmp.KhplXQz0Am-XrtSmM4b/lib/python3.10/site-packages/uvicorn/protocols/http/h11_impl.py", line 373, in run_asgi
    result = await app(self.scope, self.receive, self.send)
  File "/home/guest/.local/share/virtualenvs/tmp.KhplXQz0Am-XrtSmM4b/lib/python3.10/site-packages/uvicorn/middleware/proxy_headers.py", line 75, in __call__
    return await self.app(scope, receive, send)
  File "/home/guest/.local/share/virtualenvs/tmp.KhplXQz0Am-XrtSmM4b/lib/python3.10/site-packages/fastapi/applications.py", line 208, in __call__
    await super().__call__(scope, receive, send)
  File "/home/guest/.local/share/virtualenvs/tmp.KhplXQz0Am-XrtSmM4b/lib/python3.10/site-packages/starlette/applications.py", line 112, in __call__
    await self.middleware_stack(scope, receive, send)
  File "/home/guest/.local/share/virtualenvs/tmp.KhplXQz0Am-XrtSmM4b/lib/python3.10/site-packages/starlette/middleware/errors.py", line 181, in __call__
    raise exc
  File "/home/guest/.local/share/virtualenvs/tmp.KhplXQz0Am-XrtSmM4b/lib/python3.10/site-packages/starlette/middleware/errors.py", line 159, in __call__
    await self.app(scope, receive, _send)
  File "/home/guest/.local/share/virtualenvs/tmp.KhplXQz0Am-XrtSmM4b/lib/python3.10/site-packages/starlette/exceptions.py", line 82, in __call__
    raise exc
  File "/home/guest/.local/share/virtualenvs/tmp.KhplXQz0Am-XrtSmM4b/lib/python3.10/site-packages/starlette/exceptions.py", line 71, in __call__
    await self.app(scope, receive, sender)
  File "/home/guest/.local/share/virtualenvs/tmp.KhplXQz0Am-XrtSmM4b/lib/python3.10/site-packages/starlette/routing.py", line 656, in __call__
    await route.handle(scope, receive, send)
  File "/home/guest/.local/share/virtualenvs/tmp.KhplXQz0Am-XrtSmM4b/lib/python3.10/site-packages/starlette/routing.py", line 259, in handle
    await self.app(scope, receive, send)
  File "/home/guest/.local/share/virtualenvs/tmp.KhplXQz0Am-XrtSmM4b/lib/python3.10/site-packages/starlette/routing.py", line 61, in app
    response = await func(request)
  File "/home/guest/.local/share/virtualenvs/tmp.KhplXQz0Am-XrtSmM4b/lib/python3.10/site-packages/fastapi/routing.py", line 234, in app
    response_data = await serialize_response(
  File "/home/guest/.local/share/virtualenvs/tmp.KhplXQz0Am-XrtSmM4b/lib/python3.10/site-packages/fastapi/routing.py", line 137, in serialize_response
    raise ValidationError(errors, field.type_)
pydantic.error_wrappers.ValidationError: 1 validation error for Box
response -> item
  value is not a valid dict (type=type_error.dict)

Operating System

Linux

Operating System Details

No response

FastAPI Version

0.71.0

Python Version

3.10.1

Additional Context

No response

[databasedav]

this is a bug https://github.com/samuelcolvin/pydantic/issues/3764

[harunyasar]

Yes, I did. Like I explained above, those were my findings and I believe _prepare_response_content might need an improvement. If you are going to create a PR that can fix the issue it would be great and I would be glad to see. Apart from that, until having seen a better explanation I don’t have much to say.