If we're already sorted by the field then the sort query
        returned reverses the sort order.

def get_next_sort_string(self, field):
        """
        If we're already sorted by the field then the sort query
        returned reverses the sort order.
        """
        # self.sort_field is the currect sort 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)