class CustomModelAdmin:

    def formfield_for_manytomany(self, db_field, request=None, **kwargs):
        """
        Get a form Field for a ManyToManyField.
        """
if not db_field.rel.through._meta.auto_created:
    return None
db = kwargs.get('using')
if db_field.name in self.raw_id_fields:
    kwargs['widget'] = widgets.ManyToManyRawIdWidget(db_field.rel, using=db)
In django-admin, how can I set filter_horizontal as the default?