I found that in one of my programs I had forgotten to call session.BeginTransaction() and tx.Commit(). Effectively the sequence of calls was as follows:

session = sessionFactory.OpenSession();
// Now do some saves and updates
session.Close();

In spite of not calling BeginTransaction() and Commit(), objects were being created and updated properly. Why is this happening? Does NHibernate implicitly create transactions for me?

P.S. I am using SQL Server 2000 as my database

Sorry, it appears that only saves were working, not updates. But it is still surprising that saves were working.

You were using an identity (or native) identifier generator, so NH had to save the object right away before commit, to give you back its identifier.

Aha, so that explains it! Thx.