perform atomic operations on elements inside a container

In my DB Tshirts, Jeans and Shoes can be placed inside a Box and they can have a color that may be null. Elements of the box must have the same color. To ensure this I use a trigger: /* If I try to INSERT (OR UPDATE) an item in a box containing elements of different color, it raises an exception. */ BEGIN IF ( SELECT color FROM tshirt WHERE tshirt.box_id = NEW.box_id UNION SELECT color FROM jeans WHERE jeans.box_id = NEW.box_id UNION SELECT color FROM shoes WHERE shoes.box_id = NEW.box_id ) NEW.color THEN RAISE EXCEPTION 'Error..'; RETURN NULL; END IF; END; Suppose I want to change the color from 'Blue' to 'Red' for items inside Box 1. This code will fail because of the trigger: UPDATE Tshirts SET color = 'Red' WHERE box_id = 1; UPDATE Jeans SET color = 'Red' WHERE box_id = 1; UPDATE Shoes SET color = 'Red' WHERE box_id = 1; The only way I can change the color of items in the box is: 1. Remove all items from the box, setting their box_id to null. 2. Change the color of all the elements. 3. Put all the elements inside the box, setting back their box_id. How can I change the color of items in a box more simply and atomically through modeling or triggers?