How does NHibernate determine whether to Insert or Update a record?

Question

When using Session.SaveOrUpdate(myEntity); how does NHibernate decide how whether to insert a new record or update an existing one?

I am having trouble whilst saving one object in a S#arp project. It is retrieved from storage, then stored in session state for a couple of web requests, then saved back to the database with one property changed (not the S#arp [DomainSignature]).

I have, at runtime, compared the object that is about to be persisted with a freshly retrieved version straight from the database using the Equals() method and that returns true. However, the object still ends up creating a new row in the database.

Elsewhere in the application this is working fine but I am hoping for a pointer on how NHib is working this out.

Answer 1

Basically SaveOrUpdate() is looking for an identifier. If the identifier is present, it will update the record in the database. If the identifier is not present, it will create a new record.

However, it sounds like you might have something funky going on in your session. You might want to try SaveOrUpdateCopy() to see if this solves your issue.