          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)
 
</s> remove         if allow_delete and not many:
            raise ValueError('allow_delete should only be used for bulk updates, but you have not set many=True')
 </s> add         if allow_add_remove and not many:
            raise ValueError('allow_add_remove should only be used for bulk updates, but you have not set many=True') </s> remove                     # TODO: Following will be needed for reverse FK
                    # if field.many:
                    #     # Nested reverse fk relationship
                    #     for related_item in related:
                    #         fk_field = obj._meta.get_field_by_name(accessor_name)[0].field.name
                    #         setattr(related_item, fk_field, obj)
                    #         self.save_object(related_item)
                    # else:
                    # Nested reverse one-one relationship
                    fk_field = obj._meta.get_field_by_name(accessor_name)[0].field.name
                    setattr(related, fk_field, obj)
                    self.save_object(related)
 </s> add                     if field.many:
                        # Nested reverse fk relationship
                        for related_item in related:
                            fk_field = obj._meta.get_field_by_name(accessor_name)[0].field.name
                            setattr(related_item, fk_field, obj)
                            self.save_object(related_item)
                    else:
                        # Nested reverse one-one relationship
                        fk_field = obj._meta.get_field_by_name(accessor_name)[0].field.name
                        setattr(related, fk_field, obj)
                        self.save_object(related)
        # Deleted related objects
        if self._deleted:
            removed_relations = list(self._deleted)

          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)
keep keep keep add keep keep keep keep
 one-many writable nested modelserializer support 
rest_framework/serializers.py