BP_Group_Member_Query::set_orderby( BP_User_Query $query )

	public function set_orderby( $query ) {
		$gm_ids = $this->get_group_member_ids();
		if ( empty( $gm_ids ) ) {
			$gm_ids = array( 0 );
		}

		// For 'last_joined', 'first_joined', and 'group_activity'
		// types, we override the default orderby clause of
		// BP_User_Query. In the case of 'group_activity', we perform
		// a separate query to get the necessary order. In the case of
		// 'last_joined' and 'first_joined', we can trust the order of
		// results from  BP_Group_Member_Query::get_group_members().
		// In all other cases, we fall through and let BP_User_Query
		// do its own (non-group-specific) ordering.
		if ( in_array( $query->query_vars['type'], array( 'last_joined', 'first_joined', 'group_activity', 'group_role' ) ) ) {

			// Group Activity DESC.
			if ( 'group_activity' == $query->query_vars['type'] ) {
				$gm_ids = $this->get_gm_ids_ordered_by_activity( $query, $gm_ids );
			}

			// The first param in the FIELD() clause is the sort column id.
			$gm_ids = array_merge( array( 'u.id' ), wp_parse_id_list( $gm_ids ) );
			$gm_ids_sql = implode( ',', $gm_ids );

			$query->uid_clauses['orderby'] = "ORDER BY FIELD(" . $gm_ids_sql . ")";
		}

		// Prevent this filter from running on future BP_User_Query
		// instances on the same page.
		remove_action( 'bp_pre_user_query', array( $this, 'set_orderby' ) );
	}