code: def get_sort_indicator(self, field):
          
          indicator = ''
          if field == self.sort_field:
              indicator = 'sort-asc'
              if self.sort_order == '-':
                  indicator = 'sort-desc'
          return indicator
docs: Returns a sort class for the active sort only. That is, if field is not
          sort_field, then nothing will be returned becaues the sort is not
          active.