fix: Parsing of custom root models

changes/4883-gou177.md

@@ -0,0 +1 @@
+fix parsing of custom root models

pydantic/main.py

@@ -508,6 +508,7 @@ def json(
     def _enforce_dict_if_root(cls, obj: Any) -> Any:
         if cls.__custom_root_type__ and (
             not (isinstance(obj, dict) and obj.keys() == {ROOT_KEY})
+            and not (isinstance(obj, BaseModel) and obj.__fields__.keys() == {ROOT_KEY})
             or cls.__fields__[ROOT_KEY].shape in MAPPING_LIKE_SHAPES
         ):
             return {ROOT_KEY: obj}

tests/test_parse.py

@@ -46,6 +46,7 @@ class MyModel(BaseModel):
     m = MyModel.parse_obj('a')
     assert m.dict() == {'__root__': 'a'}
     assert m.__root__ == 'a'
+    assert MyModel.parse_obj(m) == m
 
 
 def test_parse_root_list():