Hi Al,
I read your post again, and find out you had already point out the key of this problem. I am sorry for not look at this before.
Because hibernate will insert null for id_ column then update it to the related value, seems DB2 didn't allow unique column with multiple column which value is null, as well as sql server. So I think maybe your solution is right. I will go back and have a try. Thank you for your patient.