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