fix: validation with a BaseModel field and a custom root type

changes/2449-PrettyWood.md

@@ -0,0 +1 @@
+fix validation with a `BaseModel` field and a custom root type

pydantic/main.py

@@ -717,11 +717,12 @@ def __get_validators__(cls) -> 'CallableGenerator':
 
     @classmethod
     def validate(cls: Type['Model'], value: Any) -> 'Model':
+        if isinstance(value, cls):
+            return value.copy() if cls.__config__.copy_on_model_validation else value
+
         value = cls._enforce_dict_if_root(value)
         if isinstance(value, dict):
             return cls(**value)
-        elif isinstance(value, cls):
-            return value.copy() if cls.__config__.copy_on_model_validation else value
         elif cls.__config__.orm_mode:
             return cls.from_orm(value)
         else:

tests/test_main.py

@@ -17,6 +17,7 @@
     ValidationError,
     constr,
     root_validator,
+    validate_model,
     validator,
 )
 from pydantic.typing import Literal
@@ -1137,6 +1138,17 @@ class MyModel(BaseModel):
     assert m.__root__ == ['a']
 
 
+def test_root_nested():
+    class MyList(BaseModel):
+        __root__: List[str]
+
+    class MyModel(BaseModel):
+        my_list: MyList
+
+    my_list = MyList(__root__=['pika'])
+    assert MyModel(my_list=my_list).dict() == {'my_list': ['pika']}
+
+
 def test_encode_nested_root():
     house_dict = {'pets': ['dog', 'cats']}