refactored validation of field names and created tests

rest_framework/serializers.py

@@ -997,7 +997,7 @@ class ModelSerializer(Serializer):
         # relationships as being a special case. During updates we already
         # have an instance pk for the relationships to be associated with.
         m2m_fields = []
-        partial_update_extra_fields = self.get_partial_update_extra_fields()
+        partial_update_extra_fields = self.get_partial_update_extra_fields(info.fields.keys())
         update_fields = [*(partial_update_extra_fields or [])]
         for attr, value in validated_data.items():
             if attr in info.relations and info.relations[attr].to_many:
@@ -1633,7 +1633,7 @@ class ModelSerializer(Serializer):
 
         return validators
 
-    def get_partial_update_extra_fields(self):
+    def get_partial_update_extra_fields(self, field_names):
         partial_update_extra_fields = getattr(self.Meta, 'partial_update_extra_fields', None)
 
         if partial_update_extra_fields is not None and not isinstance(partial_update_extra_fields, (list, tuple)):
@@ -1642,10 +1642,9 @@ class ModelSerializer(Serializer):
                 type(partial_update_extra_fields).__name__
             )
 
-        fields = self.get_fields()
         if partial_update_extra_fields is not None:
             for field_name in partial_update_extra_fields:
-                assert field_name in fields, (
+                assert field_name in field_names, (
                     "The field '{field_name}' was included on serializer "
                     "{serializer_class} in the 'partial_update_extra_fields' option, but does "
                     "not match any model field.".format(

tests/test_model_serializer.py

@@ -1301,6 +1301,11 @@ class Issue6751Model(models.Model):
     char_field2 = models.CharField(max_length=100)
 
 
+class Issue2648Model(models.Model):
+    char_field = models.CharField(max_length=100)
+    updated_at = models.DateTimeField(auto_now=True, blank=True, null=True)
+
+
 @receiver(m2m_changed, sender=Issue6751Model.many_to_many.through)
 def process_issue6751model_m2m_changed(action, instance, **_):
     if action == 'post_add':
@@ -1333,3 +1338,60 @@ class Issue6751Test(TestCase):
         serializer.save()
 
         self.assertEqual(instance.char_field, 'value changed by signal')
+
+
+class Issue2648Test(TestCase):
+    def test_model_serializer_uses_partial_update_extra_fields_when_not_empty(self):
+        class TestSerializer(serializers.ModelSerializer):
+            class Meta:
+                model = Issue2648Model
+                partial_update_extra_fields = ('updated_at',)
+                fields = ('updated_at', 'char_field',)
+
+        instance = Issue2648Model.objects.create(char_field='initial value')
+        instance.updated_at = None
+        instance.save(update_fields=['updated_at'])
+
+        serializer = TestSerializer(instance=instance, data={'char_field': 'char_field updated value'}, partial=True)
+        serializer.is_valid()
+        serializer.save()
+
+        self.assertEqual(instance.char_field, 'char_field updated value')
+        self.assertIsNotNone(instance.updated_at)
+
+    def test_model_serializer_uses_partial_update_extra_fields_when_empty(self):
+        class TestSerializer(serializers.ModelSerializer):
+            class Meta:
+                model = Issue2648Model
+                partial_update_extra_fields = []
+                fields = ('updated_at', 'char_field',)
+
+        instance = Issue2648Model.objects.create(char_field='initial value')
+        instance.updated_at = None
+        instance.save(update_fields=['updated_at'])
+
+        serializer = TestSerializer(instance=instance, data={'char_field': 'char_field updated value'}, partial=True)
+        serializer.is_valid()
+        serializer.save()
+
+        self.assertEqual(instance.char_field, 'char_field updated value')
+        self.assertIsNone(instance.updated_at)
+
+    def test_model_serializer_validate_partial_update_extra_fields_are_model_fields(self):
+        class TestSerializer(serializers.ModelSerializer):
+            class Meta:
+                model = Issue2648Model
+                partial_update_extra_fields = ('missing_model_field',)
+                fields = ('updated_at', 'char_field',)
+
+        instance = Issue2648Model.objects.create(char_field='initial value')
+        serializer = TestSerializer(instance=instance, data={'char_field': 'char_field updated value'}, partial=True)
+        serializer.is_valid()
+
+        expected = (
+            "The field 'missing_model_field' was included on serializer "
+            "TestSerializer in the 'partial_update_extra_fields' option, but does "
+            "not match any model field."
+        )
+        with self.assertRaisesMessage(AssertionError, expected):
+            serializer.save()