BP_Messages_Message::get_existing_thread( array $recipient_ids, integer $sender )

	public static function get_existing_thread( $recipient_ids, $sender = 0 ) {
		global $wpdb;

		$bp = buddypress();

		// add the sender into the recipient list and order by id ascending
		$recipient_ids[] = $sender;
		$recipient_ids = array_filter(array_unique(array_values($recipient_ids)));
		sort($recipient_ids);

		$results = $wpdb->get_results( $sql = $wpdb->prepare(
			"SELECT
				r.thread_id as thread_id,
				GROUP_CONCAT(DISTINCT user_id ORDER BY user_id separator ',') as recipient_list,
				MAX(m.date_sent) AS date_sent
			FROM {$bp->messages->table_name_recipients} r
			INNER JOIN {$bp->messages->table_name_messages} m ON m.thread_id = r.thread_id
			GROUP BY r.thread_id
			HAVING recipient_list = %s
			ORDER BY date_sent DESC
			LIMIT 1
			",
			implode(',', $recipient_ids)
		) );

        if ( ! $results ) {
            return null;
        }

        $thread_id = $results[0]->thread_id;

        if ( ! $is_active_recipient = BP_Messages_Thread::is_thread_recipient( $thread_id, $sender ) ) {
            return null;
        }

        return $thread_id;
	}