fix: support properly custom root type with from_orm() (#2237)

* fix: support custom root type with `from_orm()`

* add other example

* chore: add change file

* refactor(main): use ROOT_KEY instead of __root__

changes/2237-PrettyWood.md

@@ -0,0 +1 @@
+Support custom root type (aka `__root__`) with `from_orm()`

pydantic/main.py

@@ -207,7 +207,7 @@ def prepare_config(config: Type[BaseConfig], cls_name: str) -> None:
 
 def validate_custom_root_type(fields: Dict[str, ModelField]) -> None:
     if len(fields) > 1:
-        raise ValueError('__root__ cannot be mixed with other fields')
+        raise ValueError(f'{ROOT_KEY} cannot be mixed with other fields')
 
 
 # Annotated fields can have many types like `str`, `int`, `List[str]`, `Callable`...
@@ -590,7 +590,7 @@ def parse_file(
     def from_orm(cls: Type['Model'], obj: Any) -> 'Model':
         if not cls.__config__.orm_mode:
             raise ConfigError('You must have the config attribute orm_mode=True to use from_orm')
-        obj = cls._decompose_class(obj)
+        obj = {ROOT_KEY: obj} if cls.__custom_root_type__ else cls._decompose_class(obj)
         m = cls.__new__(cls)
         values, fields_set, validation_error = validate_model(cls, obj)
         if validation_error:
@@ -726,8 +726,8 @@ def _get_value(
                     exclude=exclude,
                     exclude_none=exclude_none,
                 )
-                if '__root__' in v_dict:
-                    return v_dict['__root__']
+                if ROOT_KEY in v_dict:
+                    return v_dict[ROOT_KEY]
                 return v_dict
             else:
                 return v.copy(include=include, exclude=exclude)

tests/test_orm_mode.py

@@ -1,4 +1,4 @@
-from typing import Any, List
+from typing import Any, Dict, List
 
 import pytest
 
@@ -47,6 +47,47 @@ def __getattr__(self, key):
     assert repr(gd) == "GetterDict[TestCls]({'a': 1, 'c': 3, 'd': 4})"
 
 
+def test_orm_mode_root():
+    class PokemonCls:
+        def __init__(self, *, en_name: str, jp_name: str):
+            self.en_name = en_name
+            self.jp_name = jp_name
+
+    class Pokemon(BaseModel):
+        en_name: str
+        jp_name: str
+
+        class Config:
+            orm_mode = True
+
+    class PokemonList(BaseModel):
+        __root__: List[Pokemon]
+
+        class Config:
+            orm_mode = True
+
+    pika = PokemonCls(en_name='Pikachu', jp_name='ピカチュウ')
+    bulbi = PokemonCls(en_name='Bulbasaur', jp_name='フシギダネ')
+
+    pokemons = PokemonList.from_orm([pika, bulbi])
+    assert pokemons.__root__ == [
+        Pokemon(en_name='Pikachu', jp_name='ピカチュウ'),
+        Pokemon(en_name='Bulbasaur', jp_name='フシギダネ'),
+    ]
+
+    class PokemonDict(BaseModel):
+        __root__: Dict[str, Pokemon]
+
+        class Config:
+            orm_mode = True
+
+    pokemons = PokemonDict.from_orm({'pika': pika, 'bulbi': bulbi})
+    assert pokemons.__root__ == {
+        'pika': Pokemon(en_name='Pikachu', jp_name='ピカチュウ'),
+        'bulbi': Pokemon(en_name='Bulbasaur', jp_name='フシギダネ'),
+    }
+
+
 def test_orm_mode():
     class PetCls:
         def __init__(self, *, name: str, species: str):