Stores the number of comments. A custom ``count_filter``
        queryset gets checked for, allowing managers to implement
        custom count logic.

def related_items_changed(self, instance, related_manager):
        """
        Stores the number of comments. A custom ``count_filter``
        queryset gets checked for, allowing managers to implement
        custom count logic.
        """
        try:
            count = related_manager.count_queryset()
        except AttributeError:
            count = related_manager.count()
        count_field_name = list(self.fields.keys())[0] % \
                           self.related_field_name
        setattr(instance, count_field_name, count)
        instance.save()