🐛 Fix tagged discriminated union not recognized as body field #12942

frankie567 · 2024-11-15T15:43:59Z

Problem

When using a union type models with a tagged discriminator as an endpoint parameter, like the following:

class Pie(BaseModel):
    time_to_cook: int
    num_ingredients: int


class ApplePie(Pie):
    fruit: Literal["apple"] = "apple"


class PumpkinPie(Pie):
    filling: Literal["pumpkin"] = "pumpkin"


def get_discriminator_value(v: Any) -> str:
    if isinstance(v, dict):
        return v.get("fruit", v.get("filling"))
    return getattr(v, "fruit", getattr(v, "filling", None))


Dessert = Annotated[
    Annotated[ApplePie, Tag("apple")] | Annotated[PumpkinPie, Tag("pumpkin")],
    Discriminator(get_discriminator_value),
]


app = FastAPI()
@app.post("/dessert")
async def dinner(dessert: Dessert) -> dict[str, Any]:
    return dessert.model_dump()

FastAPI fails to identify it correctly as body payload. It incorrectly considers it's a query parameter, which then causes a validation error. The workaround currently is to explicitly tell FastAPI it's a body parameter using dessert: Annotated[Dessert, Body()]

Fix

The solution is to improve the function handling type annotations by handling the case where we can have Annotated
types inside an Annotated type.

Discussed in #12941

frankie567 · 2025-05-22T07:45:20Z

Rebased/updated and modified the original comment to better explain the purpose of this PR 😄

YuriiMotov

LGTM

Actually, this problem is related to Pydantic V1 as well.
It can be tested using the following dummy test (in details)

Details

from typing import Annotated, Union

from fastapi import FastAPI
from fastapi.testclient import TestClient
from pydantic import BaseModel, Field


class Item(BaseModel):
    value: str

ItemUnion = Union[
    Annotated[Item, Field()],
    Annotated[Item, Field()],
]

app = FastAPI()


@app.post("/items/")
def save(item: ItemUnion): # Changing this to `item: ItemUnion = Body()` would fix
    return {"item": item}

# Tests


def test_() -> None:
    client = TestClient(app)
    response = client.post("/items/", json={"value": "item"})
    assert response.status_code == 200, response.text
    assert response.json() == {"item": {"value": "item"}}

But I don't think we need to fix this for Pydantic V1 now. I can't imagine any real-life use case with Pydantic V1.

patrick91

Just tested this in our codebase and it works!

I also checked that the new tests were not passing on the main branch 😊

EDIT: Also updated the test to make sure query params still work when using Annotated

patrick91 · 2025-10-08T09:29:08Z

@frankie567 I've update this to use a different style of defining the discriminator, which also makes the OpenAPI schema better 😊

Previous schema:

{
    "openapi": "3.1.0",
    "info": {"title": "FastAPI", "version": "0.1.0"},
    "paths": {
        "/items/": {
            "post": {
                "summary": "Save Union Body Discriminator",
                "operationId": "save_union_body_discriminator_items__post",
                "requestBody": {
                    "content": {
                        "application/json": {
                            "schema": {
                                "oneOf": [
                                    {"$ref": "#/components/schemas/FirstItem"},
                                    {"$ref": "#/components/schemas/OtherItem"},
                                ],
                                "title": "Item",  # No discriminator here
                            }
                        }
                    },
                    "required": True,
                },
                "responses": {
                    "200": {
                        "description": "Successful Response",
                        "content": {
                            "application/json": {
                                "schema": {
                                    "additionalProperties": True,
                                    "type": "object",
                                    "title": "Response Save Union Body Discriminator Items  Post",
                                }
                            }
                        },
                    },
                    "422": {
                        "description": "Validation Error",
                        "content": {
                            "application/json": {
                                "schema": {
                                    "$ref": "#/components/schemas/HTTPValidationError"
                                }
                            }
                        },
                    },
                },
            }
        }
    },
    "components": {
        "schemas": {
            "FirstItem": {
                "properties": {
                    "value": {"type": "string", "title": "Value"},
                    "price": {"type": "integer", "title": "Price"},
                },
                "type": "object",
                "required": ["value", "price"],
                "title": "FirstItem",
            },
            "HTTPValidationError": {
                "properties": {
                    "detail": {
                        "items": {"$ref": "#/components/schemas/ValidationError"},
                        "type": "array",
                        "title": "Detail",
                    }
                },
                "type": "object",
                "title": "HTTPValidationError",
            },
            "OtherItem": {
                "properties": {
                    "value": {"type": "string", "title": "Value"},
                    "price": {"type": "number", "title": "Price"},
                },
                "type": "object",
                "required": ["value", "price"],
                "title": "OtherItem",
            },
            "ValidationError": {
                "properties": {
                    "loc": {
                        "items": {"anyOf": [{"type": "string"}, {"type": "integer"}]},
                        "type": "array",
                        "title": "Location",
                    },
                    "msg": {"type": "string", "title": "Message"},
                    "type": {"type": "string", "title": "Error Type"},
                },
                "type": "object",
                "required": ["loc", "msg", "type"],
                "title": "ValidationError",
            },
        }
    },
}

Current schema

{
    "openapi": "3.1.0",
    "info": {"title": "FastAPI", "version": "0.1.0"},
    "paths": {
        "/items/": {
            "post": {
                "summary": "Save Union Body Discriminator",
                "operationId": "save_union_body_discriminator_items__post",
                "parameters": [
                    {
                        "name": "q",
                        "in": "query",
                        "required": True,
                        "schema": {
                            "type": "string",
                            "description": "Query string",
                            "title": "Q",
                        },
                    }
                ],
                "requestBody": {
                    "required": True,
                    "content": {
                        "application/json": {
                            "schema": {
                                "oneOf": [
                                    {"$ref": "#/components/schemas/FirstItem"},
                                    {"$ref": "#/components/schemas/OtherItem"},
                                ],
                                "discriminator": { # 👈 Discriminator
                                    "propertyName": "value",
                                    "mapping": {
                                        "first": "#/components/schemas/FirstItem",
                                        "other": "#/components/schemas/OtherItem",
                                    },
                                },
                                "title": "Item",
                            }
                        }
                    },
                },
                "responses": {
                    "200": {
                        "description": "Successful Response",
                        "content": {
                            "application/json": {
                                "schema": {
                                    "type": "object",
                                    "additionalProperties": True,
                                    "title": "Response Save Union Body Discriminator Items  Post",
                                }
                            }
                        },
                    },
                    "422": {
                        "description": "Validation Error",
                        "content": {
                            "application/json": {
                                "schema": {
                                    "$ref": "#/components/schemas/HTTPValidationError"
                                }
                            }
                        },
                    },
                },
            }
        }
    },
    "components": {
        "schemas": {
            "FirstItem": {
                "properties": {
                    "value": {"type": "string", "const": "first", "title": "Value"},
                    "price": {"type": "integer", "title": "Price"},
                },
                "type": "object",
                "required": ["value", "price"],
                "title": "FirstItem",
            },
            "HTTPValidationError": {
                "properties": {
                    "detail": {
                        "items": {"$ref": "#/components/schemas/ValidationError"},
                        "type": "array",
                        "title": "Detail",
                    }
                },
                "type": "object",
                "title": "HTTPValidationError",
            },
            "OtherItem": {
                "properties": {
                    "value": {"type": "string", "const": "other", "title": "Value"},
                    "price": {"type": "number", "title": "Price"},
                },
                "type": "object",
                "required": ["value", "price"],
                "title": "OtherItem",
            },
            "ValidationError": {
                "properties": {
                    "loc": {
                        "items": {"anyOf": [{"type": "string"}, {"type": "integer"}]},
                        "type": "array",
                        "title": "Location",
                    },
                    "msg": {"type": "string", "title": "Message"},
                    "type": {"type": "string", "title": "Error Type"},
                },
                "type": "object",
                "required": ["loc", "msg", "type"],
                "title": "ValidationError",
            },
        }
    },
}

but the code works in both cases, the fix is mostly around using Annotated :)

frankie567 · 2025-10-08T10:50:19Z

Thank you @patrick91, good point 👍

tiangolo

Cool, thanks @frankie567! 🚀

And thanks @YuriiMotov and @patrick91 for the help!

I realized that the tests were also passing on master 😅

So, I updated them to make them fail on master (adding the Tag) and get fixed by this change.

This will be released in FastAPI 0.118.2 in the next hours. 🎉

~~Ups, it seems I broke the tests, let me check that first.~~ Fixed. 😅

AlexanderFarkas · 2026-01-09T16:38:28Z

@YuriiMotov No, I don't think that's the case. I have a similar problem:

class CommunicateViaSmsDTO(BaseModel):
    message: str
    scheduled_at: Optional[datetime.datetime] = None

@dataclass
class SearchCustomerDTO:
    order_by: Optional[OrderByStatement] = None
    phone_number: Optional[str] = None
    max_last_active_datetime: Optional[datetime.datetime] = None
    
@communicate_router.post("/via_sms")
def communicate_via_sms(
    dto: CommunicateViaSmsDTO,
    search_params: SearchCustomerDTO = Depends(),
    customer_ids: list[int] = Query(default=None),
) -> APIResult[SendBatchMessageResultDTO]:
    # Implementation
    pass

And after upgrading to FastAPI 118.2 from 118.1, FastAPI doesn't see the body I send:

 admin_client = TestClient(.....)
 admin_client.post(
            f"/customers/communicate/via_email?max_last_active_datetime={last_active_datetime}",
            json={
                "subject": "hello",
                "message": "world",
            },
        )

It throws:

 AssertionError: {"ok":false,"error":{"type":"RequestValidationError","message":"[{'type': 'missing', 'loc': ('body', 'dto'), 'msg': 'Field required', 'input': None}]","errors":["{'type': 'missing', 'loc': ('body', 'dto'), 'msg': 'Field required', 'input': None}"]}}

For some reason it treats the dto as a scalar of the body (judging by the error) and changing the testing code to:

 admin_client.post(
           f"/customers/communicate/via_email?max_last_active_datetime={last_active_datetime}",
           json={
               "dto": {
                   "subject": "hello",
                   "message": "world",
               }
           },
       )

Fixes the problem

YuriiMotov · 2026-01-09T16:55:39Z

For some reason it treats the dto as a scalar of the body (judging by the error) and changing the testing code to ...
Fixes the problem

That's because FASTAPI treats one of other parameters (likely order_by: Optional[OrderByStatement]) as Body parameter. When you have several Body parameter models (or model + single Body parameter), FASTAPI embeds them

See https://fastapi.tiangolo.com/tutorial/body-multiple-params/#multiple-body-parameters

AlexanderFarkas · 2026-01-09T19:17:22Z

@YuriiMotov But it used to work correctly. Why does it treat an order_by (which is located in the SearchCustomerDTO accessed via Depends()) as body?

Here is the order_by definition:

from typing import Annotated, TypeVar, Sequence, Generic, Type, Literal

import pydantic
from fastapi import Query
from pydantic import AfterValidator, PlainValidator, WithJsonSchema, ConfigDict

OrderByList = list[tuple[str, Literal["desc", "asc"]]]


def _validate_order_by_str(v):
    if isinstance(v, list):
        couples = v
    elif isinstance(v, str):
        couples = [couple.split(":") for couple in v.split(";")]
    else:
        raise TypeError()

    result = []
    for couple in couples:
        column, order = couple

        if not isinstance(column, str):
            raise TypeError("column should be str")
        if order not in ("desc", "asc"):
            raise TypeError("Order should be either desc or asc")
        result.append(couple)
    return result


OrderByStatement = Annotated[
    OrderByList,
    PlainValidator(_validate_order_by_str),
    WithJsonSchema(
        {
            "pattern": "column_name:desc|asc;",
            "examples": ["amount:desc", "sum:asc;experience:desc"],
            "type": "string",
        },
        mode="validation",
    ),
]

Marking order_by as Query doesn't help:

order_by: Annotated[Optional[OrderByStatement], Query] = None

But you're right - commenting out order_by fixes the issue. Due to the changes made in this PR, OrderByStatement is now considered complex. Though it's not clear how to bring back the old behavior.

AlexanderFarkas · 2026-01-09T21:04:42Z

The fix was:

if TYPE_CHECKING:
    OrderByStatement = OrderByList
else:
    OrderByStatement = Annotated[
        str,
        PlainValidator(_validate_order_by_str),
        WithJsonSchema(
            {
                "pattern": "column_name:desc|asc;",
                "examples": ["amount:desc", "sum:asc;experience:desc"],
                "type": "string",
            },
            mode="validation",
        ),
    ]

I'd personally mention it as a breaking change in the changelog

frankie567 force-pushed the fix/12941 branch 2 times, most recently from 868e06f to 8b292f1 Compare November 15, 2024 15:51

svlandeg changed the title ~~Fix tagged discriminated union not recognized as body field~~ 🐛 Fix tagged discriminated union not recognized as body field Nov 25, 2024

svlandeg added the bug Something isn't working label Nov 25, 2024

frankie567 force-pushed the fix/12941 branch 2 times, most recently from 1fdf0b6 to 83e02fd Compare May 22, 2025 07:38

Fix tagged discriminated union not recognized as body field

941efda

frankie567 force-pushed the fix/12941 branch from 83e02fd to 941efda Compare May 22, 2025 07:43

YuriiMotov approved these changes Jul 8, 2025

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YuriiMotov and others added 2 commits July 8, 2025 21:24

Merge branch 'master' into fix/12941

8639ed5

Merge branch 'master' into fix/12941

09f3cba

patrick91 approved these changes Oct 7, 2025

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patrick91 added 7 commits October 7, 2025 14:22

Add test for annotated and query param

54fc233

Add test for schema, run tests on pydantic 1

891ac6e

Fix test, use discriminator on field

f95dd6c

Remove usage of tag

292592f

Test schema in both pydantic versions

b4f48d1

Fix test

b0894fd

Merge branch 'master' into fix/12941

2eb955f

✅ Refactor test to fail on master

a8f6321

tiangolo approved these changes Oct 8, 2025

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tiangolo added 3 commits October 8, 2025 16:31

♻️ Import Pydantic v2 specifics only in Pydantic v2 tests

7d51940

✅ Update tests to handle Python 3.8

d97415b

Merge branch 'master' into fix/12941

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tiangolo enabled auto-merge (squash) October 8, 2025 14:46

tiangolo merged commit 185cecd into fastapi:master Oct 8, 2025
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javitonino mentioned this pull request Oct 30, 2025

Update starlette/fastapi nuclia/nucliadb#3353

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retwish mentioned this pull request Dec 9, 2025

🚸 Improve error message for invalid query parameter type annotations #14479

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🐛 Fix tagged discriminated union not recognized as body field #12942

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