for Version 2.1.8

Document says 16.4. Using cascading update()
//parent and child were both loaded in a previous session
parent.addChild(child);
Child newChild = new Child();
parent.addChild(newChild);
session.update(parent);
session.flush();

but I am getting "net.sf.hibernate.StaleObjectStateException An operation failed due to stale data"

net.sf.hibernate.StaleObjectStateException An operation failed due to stale data
[6/22/05 12:17:30:241 EDT] 4e7d772a StaleObjectSt W net.sf.hibernate.StaleObjectStateException TRAS0014I: The following exception was logged net.sf.hibernate.StaleObjectStateException: Row was updated or deleted by another transaction (or unsaved-value mapping was incorrect) for efi.edcs.dto.Case instance with identifier: 888888

Here is the Code
// Add new Child
// got parent from previous session
if ( parent != null ) {
System.out.println("In After session close");
i = parent.getChild().size();
System.out.println("Folder Num = "+ parent.toString());
System.out.println("child Size = "+ i);
}

session = HibernateSessionFactory.currentSession();
tx = session.beginTransaction();
// add child Data
Child child = new Child();
child.setId("888888");
child.setParent(parent);

if (parent.getChild() == null) {
parent.setChild(new HashSet());
}
parent.getChild().add(child);
System.out.println("Case count : " + parent.getChild().size());

session.update(parent);
session.flush();
tx.commit();
session.close();


<id name="id" column="id" type="xxx.xxx.persistence.StringType"
unsaved-value="-1" >
<generator class="assigned" />

<id name="id" column="id" type="xxx.xxx.persistence.StringType" unsaved-value="-1" >
<generator class="assigned" />
</id>



Only one thing I am missing is

define unsaved-value="null" or unsaved-value="negative" on a <version> or <timestamp> property mapping for the class.