def _sort_to_str(self):
        params_list = []
        timestamp = ""
        for k, v in self._solr_params['sort'].items():
            if k != "timestamp":
                params_list.append(" ".join([k, v]))
            else:
                timestamp = v
        params_list.append(" ".join(['timestamp', timestamp]))
        self._solr_params['sort'] = ", ".join(params_list)