fix: Propagate Pydantic's model config

fastapi/_compat.py

@@ -87,6 +87,7 @@ if PYDANTIC_V2:
     class ModelField:
         field_info: FieldInfo
         name: str
+        model_config: BaseConfig | None = None
         mode: Literal["validation", "serialization"] = "validation"
 
         @property
@@ -108,7 +109,8 @@ if PYDANTIC_V2:
 
         def __post_init__(self) -> None:
             self._type_adapter: TypeAdapter[Any] = TypeAdapter(
-                Annotated[self.field_info.annotation, self.field_info]
+                Annotated[self.field_info.annotation, self.field_info],
+                config=self.model_config,
             )
 
         def get_default(self) -> Any:
@@ -282,7 +284,9 @@ if PYDANTIC_V2:
 
     def get_model_fields(model: Type[BaseModel]) -> List[ModelField]:
         return [
-            ModelField(field_info=field_info, name=name)
+            ModelField(
+                field_info=field_info, name=name, model_config=model.model_config
+            )
             for name, field_info in model.model_fields.items()
         ]
 

tests/test_compat.py

@@ -80,6 +80,34 @@ def test_complex():
     assert response2.json() == [1, 2]
 
 
+def test_propagates_pydantic_model_config():
+    app = FastAPI()
+
+    class Missing:
+        def __bool__(self):
+            return False
+
+    class Model(BaseModel):
+        model_config = ConfigDict(
+            arbitrary_types_allowed=True,
+        )
+        value: str | Missing = Missing()
+
+    @app.post("/")
+    def foo(req: Model) -> str | None:
+        return req.value or None
+
+    client = TestClient(app)
+
+    response = client.post("/", json={})
+    assert response.status_code == 200, response.text
+    assert response.json() is None
+
+    response2 = client.post("/", json={"value": "foo"})
+    assert response2.status_code == 200, response2.text
+    assert response2.json() == "foo"
+
+
 def test_is_bytes_sequence_annotation_union():
     # For coverage
     # TODO: in theory this would allow declaring types that could be lists of bytes