Fix nested serializer required field validation when partial=False (#2585)

rest_framework/fields.py

@@ -473,7 +473,7 @@ def get_default(self):
         raise `SkipField`, indicating that no value should be set in the
         validated data for this field.
         """
-        if self.default is empty or getattr(self.root, 'partial', False):
+        if self.default is empty or getattr(self.parent, 'partial', False):
             # No default, or this is a partial update.
             raise SkipField()
         if callable(self.default):
@@ -499,7 +499,7 @@ def validate_empty_values(self, data):
             return (True, self.get_default())
 
         if data is empty:
-            if getattr(self.root, 'partial', False):
+            if getattr(self.parent, 'partial', False):
                 raise SkipField()
             if self.required:
                 self.fail('required')

rest_framework/serializers.py

@@ -115,11 +115,21 @@ def __init__(self, instance=None, data=empty, **kwargs):
         self.instance = instance
         if data is not empty:
             self.initial_data = data
-        self.partial = kwargs.pop('partial', False)
+        self._partial = kwargs.pop('partial', None)
         self._context = kwargs.pop('context', {})
         kwargs.pop('many', None)
         super().__init__(**kwargs)
 
+    @property
+    def partial(self):
+        if self._partial is not None:
+            return self._partial
+        return getattr(self.parent, 'partial', False)
+
+    @partial.setter
+    def partial(self, value):
+        self._partial = value
+
     def __new__(cls, *args, **kwargs):
         # We override this method in order to automatically create
         # `ListSerializer` classes instead when `many=True` is set.

tests/test_serializer.py

@@ -829,3 +829,52 @@ class Meta:
         with pytest.raises(ValueError) as exc_info:
             serializer.is_valid(raise_exception=True)
         assert str(exc_info.value) == error_msg
+
+
+class TestNestedSerializerPartial:
+    def test_nested_serializer_with_partial_false(self):
+        class LocationSerializer(serializers.Serializer):
+            country = serializers.CharField()
+            city = serializers.CharField()
+
+        class UserSerializer(serializers.Serializer):
+            username = serializers.CharField()
+            location = LocationSerializer(partial=False)
+
+        # 1. When location is provided as empty dict, required fields must not be skipped
+        serializer = UserSerializer(data={'location': {}}, partial=True)
+        assert not serializer.is_valid()
+        assert serializer.errors == {
+            'location': {
+                'country': ['This field is required.'],
+                'city': ['This field is required.']
+            }
+        }
+
+        # 2. When location is provided with only one field, the other required field must not be skipped
+        serializer2 = UserSerializer(data={'location': {'city': 'Paris'}}, partial=True)
+        assert not serializer2.is_valid()
+        assert serializer2.errors == {
+            'location': {
+                'country': ['This field is required.']
+            }
+        }
+
+        # 3. When location is omitted entirely, the location field itself is skipped on partial update
+        serializer3 = UserSerializer(data={'username': 'alice'}, partial=True)
+        assert serializer3.is_valid()
+        assert serializer3.validated_data == {'username': 'alice'}
+
+    def test_nested_serializer_inherits_partial_by_default(self):
+        class LocationSerializer(serializers.Serializer):
+            country = serializers.CharField()
+            city = serializers.CharField()
+
+        class UserSerializer(serializers.Serializer):
+            username = serializers.CharField()
+            location = LocationSerializer()
+
+        # When partial is not specified on the nested serializer, it should inherit partial=True
+        serializer = UserSerializer(data={'location': {'city': 'Paris'}}, partial=True)
+        assert serializer.is_valid()
+        assert serializer.validated_data == {'location': {'city': 'Paris'}}