DROP TRIGGER IF EXISTS `message_insert`;
CREATE TRIGGER `message_insert` BEFORE INSERT ON `messages`
FOR EACH ROW BEGIN
	IF new.i_id IS NOT NULL AND new.p_id IS NOT NULL THEN
		SIGNAL SQLSTATE '45000'
		SET MESSAGE_TEXT = 'Messages can only be related to one thing';
	ELSE
		SET new.date_modified = NOW();
	END IF;
END;