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

	public static function get_existing_threads( $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
			",
				implode( ',', $recipient_ids )
			)
		);

		if ( ! $results ) {
			return null;
		}

		return $results;
	}