          You should override this method to control how deserialized objects
          are instantiated.
          """
          if instance is not None:
              instance.update(attrs)
              instance._removed_relations = removed_relations
              return instance
          return attrs
 
</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')
        removed_relations = []

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

          You should override this method to control how deserialized objects
          are instantiated.
          """
        removed_relations = []

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

          if instance is not None:
              instance.update(attrs)
              instance._removed_relations = removed_relations
              return instance
          return attrs
keep keep add keep keep keep keep keep
 one-many writable nested modelserializer support 
rest_framework/serializers.py