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

Co-authored-by: Motov Yurii <[email protected]>
Co-authored-by: Patrick Arminio <[email protected]>
Co-authored-by: Sebastián Ramírez <[email protected]>

fastapi/_compat.py

@@ -590,6 +590,9 @@ def field_annotation_is_complex(annotation: Union[Type[Any], None]) -> bool:
     if origin is Union or origin is UnionType:
         return any(field_annotation_is_complex(arg) for arg in get_args(annotation))
 
+    if origin is Annotated:
+        return field_annotation_is_complex(get_args(annotation)[0])
+
     return (
         _annotation_is_complex(annotation)
         or _annotation_is_complex(origin)

tests/test_union_body_discriminator.py

@@ -0,0 +1,188 @@
+from typing import Any, Dict, Union
+
+from dirty_equals import IsDict
+from fastapi import FastAPI
+from fastapi.testclient import TestClient
+from inline_snapshot import snapshot
+from pydantic import BaseModel, Field
+from typing_extensions import Annotated, Literal
+
+from .utils import needs_pydanticv2
+
+
+@needs_pydanticv2
+def test_discriminator_pydantic_v2() -> None:
+    from pydantic import Tag
+
+    app = FastAPI()
+
+    class FirstItem(BaseModel):
+        value: Literal["first"]
+        price: int
+
+    class OtherItem(BaseModel):
+        value: Literal["other"]
+        price: float
+
+    Item = Annotated[
+        Union[Annotated[FirstItem, Tag("first")], Annotated[OtherItem, Tag("other")]],
+        Field(discriminator="value"),
+    ]
+
+    @app.post("/items/")
+    def save_union_body_discriminator(
+        item: Item, q: Annotated[str, Field(description="Query string")]
+    ) -> Dict[str, Any]:
+        return {"item": item}
+
+    client = TestClient(app)
+    response = client.post("/items/?q=first", json={"value": "first", "price": 100})
+    assert response.status_code == 200, response.text
+    assert response.json() == {"item": {"value": "first", "price": 100}}
+
+    response = client.post("/items/?q=other", json={"value": "other", "price": 100.5})
+    assert response.status_code == 200, response.text
+    assert response.json() == {"item": {"value": "other", "price": 100.5}}
+
+    response = client.get("/openapi.json")
+    assert response.status_code == 200, response.text
+    assert response.json() == snapshot(
+        {
+            "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": {
+                                            "propertyName": "value",
+                                            "mapping": {
+                                                "first": "#/components/schemas/FirstItem",
+                                                "other": "#/components/schemas/OtherItem",
+                                            },
+                                        },
+                                        "title": "Item",
+                                    }
+                                }
+                            },
+                        },
+                        "responses": {
+                            "200": {
+                                "description": "Successful Response",
+                                "content": {
+                                    "application/json": {
+                                        "schema": IsDict(
+                                            {
+                                                # Pydantic 2.10, in Python 3.8
+                                                # TODO: remove when dropping support for Python 3.8
+                                                "type": "object",
+                                                "title": "Response Save Union Body Discriminator Items  Post",
+                                            }
+                                        )
+                                        | IsDict(
+                                            {
+                                                "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",
+                    },
+                }
+            },
+        }
+    )