Timestamp Locking with Aggregate

Hi,
Ive run into a little problem whilst upgrading to Toplink 904. I have several classes mapped to tables with timestamp locking enabled. The timestamp field is mapped through an aggregate.
If I enable Store Version in Cache at class level, new objects and their timestamps are correctly inserted to database on commit. However, the timestamps are not updated in the respective Toplink objects (they seem to be cached as null). This is problematic, as our app needs this info.
If I disable the Store Version in Cache option (which worked for us in Toplink 903), all inserts fail with the message:
2004.07.26 10:47:42.500--UnitOfWork(2086370)--Thread[main,5,main]--java.lang.NullPointerExceptionjava.lang.NullPointerException
     at oracle.toplink.internal.descriptors.ObjectBuilder.mergeChangesIntoObject(ObjectBuilder.java:1458)
     at oracle.toplink.internal.sessions.MergeManager.mergeChangesOfWorkingCopyIntoOriginal(MergeManager.java:516)
     at oracle.toplink.internal.sessions.MergeManager.mergeChanges(MergeManager.java:173)
     at oracle.toplink.publicinterface.UnitOfWork.mergeChangesIntoParent(UnitOfWork.java:2501)
     at oracle.toplink.publicinterface.UnitOfWork.commitRootUnitOfWork(UnitOfWork.java:931)
     at oracle.toplink.publicinterface.UnitOfWork.commit(UnitOfWork.java:743)
A solution is to put the timestamp field in a base class, and let the subclasses inherit it. Inserts work and timestamps are correctly updated in the Toplink objects post commit. Before I go change everything around, does anyone know why this doesnt work when the timestamp fields are mapped through aggregates in Toplink 904?
Thanks,
Steve

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Let's say there are two tables - Customer and Vehicle; both tables have the "last_update_timestamp" column.
Assuming the "timestamp locking policy" is used, if the changes to the Customer and Vehicle are in the same transaction, will the updated customer and vehicle records in database have the same timestamp after a successful commit?
Thanks.

Haiwei,
After giving your issue some more thought I wanted post an example of how you could extend TopLink shipped TimestampLockingPolicy to only get the value once per UnitOfwork. The below code is the extension and also includes and easy to use install method to add the policy into your pre-existing system. This should become the default behavior in a future release.
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package model.persistence;
import java.util.Iterator;
import oracle.toplink.descriptors.TimestampLockingPolicy;
import oracle.toplink.internal.descriptors.OptimisticLockingPolicy;
import oracle.toplink.publicinterface.Descriptor;
import oracle.toplink.queryframework.ModifyQuery;
import oracle.toplink.sessions.Session;
* This extended timestamp locking policy will perform almost identical behavior
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* @author Doug Clarke, TopLink Product Mananger
* @since TopLink 9.0.4.5 (should work on earlier versions as well)
public class MyTimestampLockingPolicy extends TimestampLockingPolicy {
/** Property key for session (UnitOfWork) properties map. */
public static final String TS_PROPERTY = "OptLock-new-timestamp";
   * This method will install this extended policy into all descriptors for the
   * given session that use the parent TimestampLockingPolicy. This is typically
   * called from a preLogin session event.
   * <code>
   *   public void preLogin(SessionEvent event) {
   *      MyTimestampLockingPolicy.install(event.getSession());
   * </code>
   * The assumption is that by calling
   * this in the preLogin the session/descriptor/policy are not yet initialized.
   * Some additional code is provided for the case that you are invoking this
   * on a session that is already initialized to ensure the policy is properly
   * configured for use.
   * @param session
public static void install(Session session) {
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      Descriptor desc = (Descriptor) i.next();
      OptimisticLockingPolicy originalPolicy = desc.getOptimisticLockingPolicy();
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      tsValue = super.getNewLockValue(query);
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   * During the insert of a new object an initial timestamp value must be
   * retrieved. This method is written similar to the getNewLockValue.
   * @param session
   * @return New timestamp value to be used in each row's insert
protected Object getInitialWriteValue(oracle.toplink.publicinterface.Session session) {
    Object tsValue = session.getProperty(TS_PROPERTY);
    if (tsValue == null) {
      tsValue = super.getInitialWriteValue(session);
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Timestamp Locking with Aggregate

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