code: def __list_uniques(self, date_range, field_name):
          
          # Get project list
          s = Search(using=self._es_conn, index=self._es_index)
          s = s.filter('range', **date_range)
          # from:to parameters (=> from: 0, size: 0)
          s = s[0:0]
          s.aggs.bucket('uniques', 'terms', field=field_name, size=1000)
          response = s.execute()
          uniques_list = []
          for item in response.aggregations.uniques.buckets:
              uniques_list.append(item.key)
  
          return uniques_list
docs: Retrieve a list of unique values in a given field within a date range.
  
          :param date_range:
          :param field_name:
          :return: list  of unique values.