Test and quick fix for #1257

rest_framework/serializers.py

@@ -893,6 +893,7 @@ class ModelSerializer(Serializer):
             field_name = field.source or field_name
             if field_name in exclusions \
                 and not field.read_only \
+                and field.required \
                 and not isinstance(field, Serializer):
                 exclusions.remove(field_name)
         return exclusions

rest_framework/tests/test_serializer.py

@@ -71,6 +71,15 @@ class ActionItemSerializer(serializers.ModelSerializer):
     class Meta:
         model = ActionItem
 
+class ActionItemSerializerOptionalFields(serializers.ModelSerializer):
+    """
+    Intended to test that fields with `required=False` are excluded from validation.
+    """
+    title = serializers.CharField(required=False)
+
+    class Meta:
+        model = ActionItem
+        fields = ('title',)
 
 class ActionItemSerializerCustomRestore(serializers.ModelSerializer):
 
@@ -288,7 +297,13 @@ class BasicTests(TestCase):
         serializer.save()
         self.assertIsNotNone(serializer.data.get('id',None), 'Model is saved. `id` should be set.')
 
-
+    def test_fields_marked_as_not_required_are_excluded_from_validation(self):
+        """
+        Check that fields with `required=False` are included in list of exclusions.
+        """
+        serializer = ActionItemSerializerOptionalFields(self.actionitem)
+        exclusions = serializer.get_validation_exclusions()
+        self.assertTrue('title' in exclusions, '`title` field was marked `required=False` and should be excluded')
 
 
 class DictStyleSerializer(serializers.Serializer):