fix: handle basemodel fallback for custom encoders

pydantic/main.py

@@ -454,6 +454,7 @@ def json(
         exclude_defaults: bool = False,
         exclude_none: bool = False,
         encoder: Optional[Callable[[Any], Any]] = None,
+        to_dict: bool = True,
         **dumps_kwargs: Any,
     ) -> str:
         """
@@ -469,11 +470,12 @@ def json(
             exclude_unset = skip_defaults
         encoder = cast(Callable[[Any], Any], encoder or self.__json_encoder__)
 
-        # We don't directly call `self.dict()`, which does exactly the same thing but
-        # with `to_dict = True` because we want to keep raw `BaseModel` instances and not as `dict`.
+        # We don't directly call `self.dict()`, which is does exactly this with `to_dict=True`
+        # because we want to be able to keep raw `BaseModel` instances and not as `dict`.
         # This allows users to write custom JSON encoders for given `BaseModel` classes.
         data = dict(
             self._iter(
+                to_dict=to_dict,
                 by_alias=by_alias,
                 include=include,
                 exclude=exclude,

tests/test_json.py

@@ -319,3 +319,41 @@ class Config:
         pumbaa.json() == '{"name": "Pumbaa", "SSN": 234, "birthday": 737424000.0, "phone": 18007267864, "friend": null}'
     )
     assert timon.json() == '{"name": "Timon", "SSN": 123, "birthday": 738892800.0, "phone": 18002752273, "friend": 234}'
+
+
+def test_custom_encode_fallback_basemodel():
+    class MyExoticType:
+        pass
+
+    def custom_encoder(o):
+        if isinstance(o, MyExoticType):
+            return 'exo'
+        raise TypeError('not serialisable')
+
+    class Foo(BaseModel):
+        x: MyExoticType
+
+        class Config:
+            arbitrary_types_allowed = True
+
+    class Bar(BaseModel):
+        foo: Foo
+
+    assert Bar(foo=Foo(x=MyExoticType())).json(encoder=custom_encoder) == '{"foo": {"x": "exo"}}'
+
+
+def test_custom_encode_error():
+    class MyExoticType:
+        pass
+
+    def custom_encoder(o):
+        raise TypeError('not serialisable')
+
+    class Foo(BaseModel):
+        x: MyExoticType
+
+        class Config:
+            arbitrary_types_allowed = True
+
+    with pytest.raises(TypeError, match='not serialisable'):
+        Foo(x=MyExoticType()).json(encoder=custom_encoder)