Called by _collection to get the final set to work on. Return the name
        of the set to use, and a list of keys to delete once the collection is
        really called (in case of a computed set based on multiple ones)

def _get_final_set(self, sets, pk, sort_options):
        """
        Called by _collection to get the final set to work on. Return the name
        of the set to use, and a list of keys to delete once the collection is
        really called (in case of a computed set based on multiple ones)
        """
        conn = self.cls.get_connection()
        all_sets = set()
        tmp_keys = set()

        if pk is not None and not sets and not (sort_options and sort_options.get('get')):
            # no final set if only a pk without values to retrieve
            return (None, False)

        elif sets or pk:
            if sets:
                new_sets, new_tmp_keys = self._prepare_sets(sets)
                all_sets.update(new_sets)
                tmp_keys.update(new_tmp_keys)
            if pk is not None:
                # create a set with the pk to do intersection (and to pass it to
                # the store command to retrieve values if needed)
                tmp_key = self._unique_key()
                conn.sadd(tmp_key, pk)
                all_sets.add(tmp_key)
                tmp_keys.add(tmp_key)

        else:
            # no sets or pk, use the whole collection instead
            all_sets.add(self.cls.get_field('pk').collection_key)

        if not all_sets:
            delete_set_later = False
            final_set = None
        elif len(all_sets) == 1:
            # if we have only one set, we  delete the set after calling
            # collection only if it's a temporary one, and we do not delete
            # it right now
            final_set = all_sets.pop()
            if final_set in tmp_keys:
                delete_set_later = True
                tmp_keys.remove(final_set)
            else:
                delete_set_later = False
        else:
            # more than one set, do an intersection on all of them in a new key
            # that will must be deleted once the collection is called.
            delete_set_later = True
            final_set = self._combine_sets(all_sets, self._unique_key())

        if tmp_keys:
            conn.delete(*tmp_keys)

        # return the final set to work on, and a flag if we later need to delete it
        return (final_set, [final_set] if delete_set_later else None)