          for field_name in attrs.keys():
              if isinstance(self.fields.get(field_name, None), Serializer):
                  nested_forward_relations[field_name] = attrs[field_name]
  
          # Deleted related objects
          if self._deleted:
              removed_relations = list(self._deleted)
  
          # Update an existing instance...
          if instance is not None:
              for key, val in attrs.items():
                  setattr(instance, key, val)
  
 
</s> remove         removed_relations = []

        # Deleted related objects
        if self._deleted:
            removed_relations = list(self._deleted)

 </s> add  </s> remove             instance._removed_relations = removed_relations
 </s> add  </s> remove         removed_relations = []
 </s> add  </s> remove                 ret = []
 </s> add                 ret = RelationsList() </s> remove         self._deleted = None
 </s> add 
          for field_name in attrs.keys():
              if isinstance(self.fields.get(field_name, None), Serializer):
                  nested_forward_relations[field_name] = attrs[field_name]
  




          # Update an existing instance...
          if instance is not None:
              for key, val in attrs.items():
                  setattr(instance, key, val)
  
keep keep keep keep replace replace replace replace keep keep keep keep keep
 Fix model serializer nestesd delete behavior 
rest_framework/serializers.py