In the book when talking about possible business keys for the Bid class the item ID along with the bid amount is mentioned. My question is if you decided to use the item ID instead of a created date I would assume you'd have to get that id by traversing the relationship to the item object in the equals() and hashcode() methods. Do you think that would affect performance enough to deter people from using associate ids? Or do you think that would be a fine business key practice?

Regards,
Mike

You can call the associated objects identifier property without initializing the associated object (if its set to lazy/proxy), so no performance hit.