NHibernate: How is identity Id updated when saving a transient instance?

Question

If I use session-per-transaction and call:

session.SaveOrUpdate(entity) corrected:
session.SaveOrUpdateCopy

Answer 1

As far as your question is concerned, whenever you flush your session is when your entity is persisted to the database. When saving your (new) entity, NHibernate generates the ID for you using the generator you provided.

Keep in mind that an Identity generator is not recommended (see this post by Ayende). When you use an Identity generator, your new entity is persisted to the database when you save, even if you don't flush to the database. The reason this happens is because NHibernate needs to provide you with an ID for the entity, which it can't do without doing a roundtrip to the database.

A better solution would be to use something like a Guid generator, or HiLo if you want 'normal' values. This way you can save your entity without actually having to do a database roundtrip, which allows you to do a lot more performance wise (batching comes to mind).