You probably don't want to use a boolean because what is a favorite for one user, might not be a favorite for another user. Try using an association and use ‘add’ in a change activity to set the association between the object to the user if you click on the favorite icon (and remove if you click the favorite icon again). To make it visible to the user that something is a favorite or not you might need a helper object to toggle a boolean and display empty star or filled star.