Should I build in referential integrity in my MySQL database (InnoDB) or let Hibernate handle it?

Some ORM solutions recommend letting the ORM handle it, not the db.

Thanks!

If you ever plan to access your data by other means than your hibernate-using application (one of the great advantages of using a rdbms as persistance storage that you can), you should have all referential integrity in the db and not only rely on the ORM.

A simple example would be if you have to import data from a legacy application. Not having integrity constraints in the DB would allow import of data that could completely mess up your application - NullPointerExceptions guaranteed.

And even if you only use Hibernate, I would recommend referential integrity.

For example let's suppose you have an object having a collection of other objects (in a Map for example). If you load one of the object in the collection via session.load() and then delete it, I am not sure Hibernate will automatically delete it from the collections as well (it is your responsability to do so, I think) and then later you will have exceptions because one of the items in the collection is no longer in the DB. With referential integrity you are protected from that.

PS: can someone confirm that indeed Hibernate won't delete it from any collection where the object may be in?

Referential integrity at the database level should never be sidestepped. You simply can't trust your Hibernate developers to get it right, every time. The integrity check at the database level will help to keep everyone honest.