Cannot return response containing pydantic model that includes aliases since FastAPI 0.47.1

Starting in 0.47.1, responses fail validation if they include any pydantic model objects that contain field aliases

You can work around this by explicitly calling .dict(by_alias=True) on the top-level response model object before returning it from the path operation function (see the example below). However, this is super unintuitive and makes the use of field aliases painful. In our codebase we use aliases extensively (via Config.alias_generator) to expose camelCase in our public APIs while keeping our internal field names snake_case, and this issue is a real hassle.

To Reproduce

1. Create a test.py file with:

import fastapi
import pydantic
import starlette.testclient
from typing import List

app = fastapi.FastAPI()

class Item(pydantic.BaseModel):
    my_value: int = pydantic.Field(..., alias='myValue')

class RootResponse(pydantic.BaseModel):
    items: List[Item] = pydantic.Field(...)

@app.get('/broken_1', response_model=RootResponse)
async def broken_1():
    # embedded models don't work if they have aliased fields
    return {'items': [Item(myValue=5), Item(myValue=10)]}

@app.get('/broken_2', response_model=RootResponse)
async def broken_2():
    # making the top-level response a model instance doesn't help
    return RootResponse(items=[Item(myValue=5), Item(myValue=10)])

@app.get('/workaround', response_model=RootResponse)
async def workaround():
    # workaround is to build a top-level model and then dict(by_alias=True)
    return RootResponse(items=[Item(myValue=5), Item(myValue=10)]).dict(by_alias=True)

with starlette.testclient.TestClient(app) as test_client:
    for path in ['/broken_1', '/broken_2', '/workaround']:
        try:
            print('-------------------------------------------')
            print(path)
            response = test_client.get(path)
        except Exception as exc:
            print(exc)
        else:
            print('succeeded')

1. Run python3 test.py with FastAPI 0.47.0. All 3 cases succeed.
2. Run python3 test.py with FastAPI 0.49.0. Only the “workaround” case succeeds.

Expected behavior

Model classes that include aliased fields (whether the alias is directly specified on the field as in the above example, or using a pydantic Config.alias_generator) should be usable with the context of a response model.

Environment

• OS: Linux (Ubuntu 18.04)
• FastAPI Version: 0.47.1 and above
• Python version: 3.7, 3.8

[jmriebold]

I’ve run into this as well. We solved it by subclassing BaseModel like so:

class CustomBaseModel(BaseModel):
    class Config:
        # Don't force use of aliases in class instantiation/attribute access
        allow_population_by_field_name = True

I tried running your test script using the subclassed model and it appeared to work correctly:

/broken_1
succeeded
-------------------------------------------
/broken_2
succeeded
-------------------------------------------
/workaround
succeeded

[github-actions[bot]]

Assuming the original issue was solved, it will be automatically closed now. But feel free to add more comments or create new issues.