Prepare the value to be stored in the zset

        We'll store the value and pk concatenated.

        For the parameters, see BaseRangeIndex.prepare_value_for_storage

def prepare_value_for_storage(self, value, pk):
        """Prepare the value to be stored in the zset

        We'll store the value and pk concatenated.

        For the parameters, see BaseRangeIndex.prepare_value_for_storage
        """
        value = super(TextRangeIndex, self).prepare_value_for_storage(value, pk)
        return self.separator.join([value, str(pk)])