code: def initial(self, request, *args, **kwargs):
          super(FlatMultipleModelMixin, self).initial(request, *args, **kwargs)
          assert not (self.sorting_field and self.sorting_fields), \
              '{} should either define ``sorting_field`` or ``sorting_fields`` property, not both.' \
              .format(self.__class__.__name__)
          if self.sorting_field:
              warnings.warn(
                  '``sorting_field`` property is pending its deprecation. Use ``sorting_fields`` instead.',
                  DeprecationWarning
              )
              self.sorting_fields = [self.sorting_field]
          self._sorting_fields = self.sorting_fields