def generate_unique_slug(self):
        """
        Create a unique slug by passing the result of get_slug() to
        utils.urls.unique_slug, which appends an index if necessary.
        """
        # For custom content types, use the ``Page`` instance for
        # slug lookup.
        concrete_model = base_concrete_model(Slugged, self)
        slug_qs = concrete_model.objects.exclude(id=self.id)
        return unique_slug(slug_qs, "slug", self.get_slug())