I'm writing a web application and am unable to get Hibernate to save new rows when using saveOrUpdate(). Existing records are updated as expected.

Here is the important part of my config file:
<hibernate-mapping>
<class name="com.datamodel.User" table="users">
<id name="id" type="integer" column="id" unsaved-value="null">
<generator class="identity">
<param name="type">integer</param>
</generator>
</id>
...

And here is the code to save/update an object of this type:

public String saveUser(User user)
{
String errorStr = null;
Session session = null;
try
{
session = sessionFactory.openSession();

Transaction transaction = session.beginTransaction();

session.saveOrUpdate(user);
transaction.commit();
session.close();
} catch (HibernateException he) {
errorStr = he.getMessage();
log.error("Unable to save user " + user.getUserId(), he);
}
return errorStr;
}

Any help would be appreciated!

set unsaved-value correctly

I have unsaved-value set according to the Hibernate documentation as far as I can tell. I'm not sure what you mean. Any more detail would be greatly appreciated.

In your case (unsaved-value=null) user will be saved (inserted) if User.id is null value.
Is user id an object (Integer, Long, etc) ? If yes, it must be null.
If not (primary type), then you must supply an appropriate unsaved value (0 for example).