DELIMITER $$
CREATE OR REPLACE DEFINER=`root`@`localhost` TRIGGER `vn`.`itemShelving_beforeUpdate`
	BEFORE UPDATE ON `itemShelving`
	FOR EACH ROW
BEGIN

	SET NEW.editorFk = account.myUser_getId();
	IF NEW.userFk IS NULL THEN
		SET NEW.userFk = account.myUser_getId();
	END IF;

	IF (NEW.visible <> OLD.visible) THEN
		SET NEW.available = GREATEST(NEW.available + NEW.visible - OLD.visible, 0);
	END IF;

END$$
DELIMITER ;