code: 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)