bbp_query_post_parent__in( string $where, WP_Query $object = '' )

function bbp_query_post_parent__in( $where, $object = '' ) {
	global $wpdb, $wp;

	// Noop if WP core supports this already
	if ( in_array( 'post_parent__in', $wp->private_query_vars ) )
		return $where;

	// Bail if no object passed
	if ( empty( $object ) )
		return $where;

	// Only 1 post_parent so return $where
	if ( is_numeric( $object->query_vars['post_parent'] ) )
		return $where;

	// Including specific post_parent's
	if ( ! empty( $object->query_vars['post_parent__in'] ) ) {
		$ids    = implode( ',', wp_parse_id_list( $object->query_vars['post_parent__in'] ) );
		$where .= " AND {$wpdb->posts}.post_parent IN ($ids)";

	// Excluding specific post_parent's
	} elseif ( ! empty( $object->query_vars['post_parent__not_in'] ) ) {
		$ids    = implode( ',', wp_parse_id_list( $object->query_vars['post_parent__not_in'] ) );
		$where .= " AND {$wpdb->posts}.post_parent NOT IN ($ids)";
	}

	// Return possibly modified $where
	return $where;
}