Multiple Responses Models

Example

Resource:

@router.get("/season/{season_id}", response_model=Union[Split, Season], response_model_exclude={'id'})
async def get_season(season_id: int, split: Optional[int] = None):
    season = await Season.get_by_season_id(season_id)
    if not season:
        return JSONResponse(status_code=404, content={"message": "Season not found"})
    if split:
        return await Split.get_by_number(season.id, split)
    else:
        return season

Models

• Season:

class Season(Model, ABC):
    season_id: Optional[int] = Field(1, ge=1)
    start_date: Optional[datetime] = datetime(date.today().year, 1, 1)
    end_date: Optional[datetime] = datetime(date.today().year, 12, 31)
    splits: Optional[List[ObjectId]] = []

    async def create(self) -> None:
        self.season_id = await Season.count_seasons() + 1
        await self.save()

    @classmethod
    async def get_by_id(cls, _id: int) -> "Season":
        return await engine.find_one(cls, cls.id == _id)

• Split:

class Split(Model, ABC):
    split_id: Optional[int] = Field(1, ge=1)
    number: Optional[int] = Field(1, ge=1)
    start_date: Optional[datetime] = datetime(date.today().year, date.today().month, 1)
    end_date: Optional[datetime]
    season: Optional[ObjectId]

    async def create(self) -> None:
        season = await Season.get_last()
        self.split_id = await self.count_splits() + 1
        self.number = season.count_splits() + 1
        self.start_date = datetime(date.today().year, date.today().month, 1)
        self.end_date = self.last_day_of_month(self.start_date)
        self.season = season.id
        await self.save()
        season.splits.append(self.id)
        await season.save()

    @classmethod
    async def get_by_number(cls, season_id, split_number: int) -> "Split":
        return await engine.find_one(cls, cls.season == season_id, cls.number == split_number)

Description

• I expect to return a season model when the query is empty, and a split model if the query is not empty
• When I try to get “/season/3?split=2” it returns

{
    "split_id": 2,
    "number": 2,
    "start_date": "2020-12-01T00:00:00",
    "end_date": "2020-12-31T00:00:00",
    "season": "5fd894151470b8ec917c6007"
}

as expected.

• But when I try to get “/season/3” it returns

{
    "split_id": 1,
    "number": 1,
    "start_date": "2020-01-01T00:00:00",
    "end_date": "2020-12-31T00:00:00",
    "season": null
}

which is still a Split Model and not a Season Model, even though the method returns a Season Model and not a Split Model.

The expected output would be:

{
    "season_id": 3,
    "start_date": "2020-01-01T00:00:00",
    "end_date": "2020-12-31T00:00:00",
    "splits": [
        "5fd894271470b8ec917c6008",
        "5fd8942e1470b8ec917c6009"
    ]
}

which I achieve if I swap Split and Season on:

@router.get("/season/{season_id}", response_model=Union[Season, Split], response_model_exclude={'id'})

So it seems that only the first model is being used.

Environment

• OS: Windows 10

• FastAPI Version: 0.62.0

• Python version: 3.9.1

Additional context

[Mause]

@hellocoldworld I think you’ll find that’s exactly why Literal exists

import json
from pydantic import BaseModel
from typing import Literal, Union

class Season(BaseModel):
    model_type: Literal['season']
    hello: int

class Split(BaseModel):
    model_type: Literal['split']
    hello: str


class Parent(BaseModel):
    thing: Union[Season, Split]

[waderoberts123]

Constructing the pydantic model object prior to returning it ensures that the data is valid, so you can directly return a JSONResponse after using the jsonable_encoder on the pydantic object.

Keep in mind that you will still want to keep the type reference in the response_model parameter so that your OpenAPI docs will still work.

References:

• https://fastapi.tiangolo.com/advanced/custom-response/
• https://fastapi.tiangolo.com/tutorial/encoder/?h=jsonable_en#using-the-jsonable_encoder