Validation Error when using response_model and Dynamic Key type

[merc]

Describe the bug

Hi guys, seem that when using a Dynamic Key type of model with response_model will raise ValidationError on the server side and returns 500 Internal Server Error to the client.

I have the following example code, I believe I am using it correctly?

could this be relater to #911 ?

I also realize I can substitute response_model=PetsByName with response_model=Dict[str, str], but with the later I am not able to include an example in schema_extra, is there other way to do so? many thanks!

To Reproduce

Steps to reproduce the behavior with a minimum self-contained file.

Replace each part with your own scenario:

1. Create a file with:

main.py

from pydantic import BaseModel
from fastapi import FastAPI
from typing import Dict

app = FastAPI()

class PetsByName(BaseModel):
    __root__: Dict[str, str]

    class Config:
        schema_extra = {
                'example': {'Otis': 'dog', 'Milo': 'cat'}
            }

@app.get("/animal/", response_model=PetsByName)
async def animal():
    return {'Otis': 'dog', 'Milo': 'cat'}

2. Using curl to test the endpoint:

$ curl -L localhost:8000/animal
Internal Server Error

3. It returns a JSON with Internal Server Error.

4. But I expected it to return {'Otis': 'dog', 'Milo': 'cat'}.

5. Terminal output of the server:

...
  File "/home/merc/.virtualenvs/py38/lib/python3.8/site-packages/starlette/routing.py", line 41, in app
    response = await func(request)
  File "/home/merc/.virtualenvs/py38/lib/python3.8/site-packages/fastapi/routing.py", line 140, in app
    response_data = await serialize_response(
  File "/home/merc/.virtualenvs/py38/lib/python3.8/site-packages/fastapi/routing.py", line 78, in serialize_response
    raise ValidationError(errors, field.type_)
pydantic.error_wrappers.ValidationError: 1 validation error for PetsByName
response -> __root__
  field required (type=value_error.missing)

Expected behavior

Add a clear and concise description of what you expected to happen.

Screenshots

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Environment

• OS: Ubuntu 19.04
• FastAPI Version: 0.48.0
• Python version: 3.8.1

Additional context

SwaggerUI renders the PetsByName model along with the example correctly, no issue there.

[merc]

After reading #911 it definitely is related to that. Will close this one and move myself over to #911 for discussion. Many thanks!

[tiangolo]

Thanks for reporting back and closing the issue. Let's continue on the other one. 👍

[github-actions]

Assuming the original need was handled, this will be automatically closed now. But feel free to add more comments or create new issues or PRs.