One of the great features of Nhibernate is that it manages persistance for us. You just attach an object to the session and NHibernate will figure out if this object is added or changed. But how does NHibernate knows the difference between a new and existing object?
By default it uses the value we assigned to the unsaved-value attribute on the id mapping. This means that if the id of our object is equal to our unsaved-value that NHibernate will detect this object as new and do an INSERT statement. If the id value is different from our unsaved-value NHibernate will generate an UPDATE statement instead.
1: <hibernate-mapping default-cascade="none" xmlns="urn:nhibernate-mapping-2.2">
2: <class name="Test.Data.Domain.Category, Test.Data" table="Categories" lazy="true">
3: <id name="CategoryID" type="System.Int32" column="CategoryID" unsaved-value="0">
4: <generator class="native" />
Sounds easy but what if you are using a composite key? In that case using the unsaved-value makes no sense. If we have a look at the documentation NHibernate gives us a second option:
A version or timestamp property should never be null for a detached instance, so Hibernate will detact any instance with a null version or timestamp as transient, no matter what other unsaved-value strategies are specified. Declaring a nullable version or timestamp property is an easy way to avoid any problems with transitive reattachment in Hibernate, especially useful for people using assigned identifiers or composite keys!
So if you leave your version column empty, NHibernate will always detect the object as new.