code: def _combine_sets(self, sets, final_set):
          
          if self._has_sortedsets:
              self.cls.get_connection().zinterstore(final_set, list(sets))
          else:
              final_set = super(ExtendedCollectionManager, self)._combine_sets(sets, final_set)
          return final_set
docs: Given a list of set, combine them to create the final set that will be
          used to make the final redis call.
          If we have a least a sorted set, use zinterstore insted of sunionstore