code: def get_next_sort_string(self, field):
          if field == self.sort_field:
              next_sort = self.toggle_sort_order() + field
          else:
              default_order_for_field = \
                  self._allowed_sort_fields[field]['default_direction']
              next_sort = default_order_for_field + field
          return self.get_sort_string(next_sort)