squash internal __root__ models in .dict() (#1607)

fix #1414

* flatten internal __root__ models

* add 1414 changes doc

changes/1414-patrickkwang.md

@@ -0,0 +1 @@
+Squash internal `__root__` dicts in `.dict()` (and, by extension, in `.json()`).

pydantic/main.py

@@ -610,14 +610,17 @@ def _get_value(
 
         if isinstance(v, BaseModel):
             if to_dict:
-                return v.dict(
+                v_dict = v.dict(
                     by_alias=by_alias,
                     exclude_unset=exclude_unset,
                     exclude_defaults=exclude_defaults,
                     include=include,
                     exclude=exclude,
                     exclude_none=exclude_none,
                 )
+                if '__root__' in v_dict:
+                    return v_dict['__root__']
+                return v_dict
             else:
                 return v.copy(include=include, exclude=exclude)
 

tests/test_main.py

@@ -869,6 +869,26 @@ class MyModel(BaseModel):
     assert m.__root__ == ['a']
 
 
+def test_encode_nested_root():
+    house_dict = {'pets': ['dog', 'cats']}
+
+    class Pets(BaseModel):
+        __root__: List[str]
+
+    class House(BaseModel):
+        pets: Pets
+
+    assert House(**house_dict).dict() == house_dict
+
+    class PetsDeep(BaseModel):
+        __root__: Pets
+
+    class HouseDeep(BaseModel):
+        pets: PetsDeep
+
+    assert HouseDeep(**house_dict).dict() == house_dict
+
+
 def test_root_failed():
     with pytest.raises(ValueError, match='__root__ cannot be mixed with other fields'):