Support custom encoder fallback on older Pydantic

fastapi/encoders.py

@@ -265,7 +265,8 @@ def jsonable_encoder(
                     sqlalchemy_safe=sqlalchemy_safe,
                 )
 
-            obj_dict = obj.model_dump(
+            obj_dict = obj.__pydantic_serializer__.to_python(
+                obj,
                 mode="json",
                 include=include,
                 exclude=exclude,
@@ -277,11 +278,8 @@ def jsonable_encoder(
             )
             return jsonable_encoder(
                 obj_dict,
-                by_alias=by_alias,
-                exclude_unset=exclude_unset,
                 exclude_none=exclude_none,
                 exclude_defaults=exclude_defaults,
-                custom_encoder=custom_encoder,
                 sqlalchemy_safe=sqlalchemy_safe,
             )
         obj_dict = obj.model_dump(

tests/test_jsonable_encoder.py

@@ -284,6 +284,21 @@ def test_custom_encoder_model_field_uses_caller_options():
     ) == {"value": {"required": 1}}
 
 
+def test_custom_encoder_model_field_does_not_encode_field_names():
+    class CustomValue:
+        pass
+
+    class ModelWithCustomValue(BaseModel):
+        model_config = ConfigDict(arbitrary_types_allowed=True)
+
+        value: CustomValue
+
+    assert jsonable_encoder(
+        ModelWithCustomValue(value=CustomValue()),
+        custom_encoder={CustomValue: lambda _: "encoded", str: str.upper},
+    ) == {"value": "ENCODED"}
+
+
 def test_custom_enum_encoders():
     def custom_enum_encoder(v: Enum):
         return v.value.lower()