The other day I used for the first time derived properties in Hibernate. What this means is that instead of mapping a property of your POJO class to a database column, your property is computed dynamically by doing an additional sub-select.
It is briefly discussed in chapter 5.1.11. of the Hibernate documentation. It is probably not a feature that will need often, however it can be a life/time-safer if you need it. It worked great for me.