I have a users table which has a field called userid. I wanted hibernate to automatically generate a unique identifier. I tried setting this as an unique identified data type using management studio but hibernate refuses to save the object. I've also tried it as an integer. It's not my hbm.xml file because it works when I set it as increment. For product codes and invoice numbers I think it looks better to use UID. Why doesn't it work?

I'm using visual studio 2008 express and sql server 2005 express