"id cannot be zero" on google appEngine

[hairinwind]

I got the exception "id cannot be zero" from google appEngine when a new record is going to be inserted.
The application is gwt + spring roo. The gwt work is done by me, not generated by spring roo. It works fine before I switch it to google app engine.

The code about id was generated by spring roo.

@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
@Column(name = "id")
private Long Item.id;

The id will be populated by the database. The google appEngine does not allow id to be zero. How does it work? Thanks,


Caused by: java.lang.IllegalArgumentException: id cannot be zero
at com.google.appengine.api.datastore.KeyFactory.crea teKey(KeyFactory.java:44)
at com.google.appengine.api.datastore.KeyFactory.crea teKey(KeyFactory.java:31)
at org.datanucleus.store.appengine.DatastoreFieldMana ger.storePrimaryKey(DatastoreFieldManager.java:760 )
at org.datanucleus.store.appengine.DatastoreFieldMana ger.storeObjectField(DatastoreFieldManager.java:78 4)
at org.datanucleus.state.AbstractStateManager.provide dObjectField(AbstractStateManager.java:1037)
at my.alarm.domain.Item.jdoProvideField(Item.java)
at my.alarm.domain.Item.jdoProvideFields(Item.java)
at org.datanucleus.state.JDOStateManagerImpl.provideF ields(JDOStateManagerImpl.java:2715)
at org.datanucleus.store.appengine.DatastorePersisten ceHandler.insertPreProcess(DatastorePersistenceHan dler.java:341)
at org.datanucleus.store.appengine.DatastorePersisten ceHandler.insertObjects(DatastorePersistenceHandle r.java:251)
at org.datanucleus.store.appengine.DatastorePersisten ceHandler.insertObject(DatastorePersistenceHandler .java:240)
at org.datanucleus.state.JDOStateManagerImpl.internal MakePersistent(JDOStateManagerImpl.java:3185)
at org.datanucleus.state.JDOStateManagerImpl.flush(JD OStateManagerImpl.java:4513)
at org.datanucleus.ObjectManagerImpl.flushInternal(Ob jectManagerImpl.java:2814)
at org.datanucleus.ObjectManagerImpl.flush(ObjectMana gerImpl.java:2754)
at org.datanucleus.ObjectManagerImpl.preCommit(Object ManagerImpl.java:2893)
at org.datanucleus.TransactionImpl.internalPreCommit( TransactionImpl.java:369)
at org.datanucleus.TransactionImpl.commit(Transaction Impl.java:256)
at org.datanucleus.jpa.EntityTransactionImpl.commit(E ntityTransactionImpl.java:104)
at org.datanucleus.store.appengine.jpa.DatastoreEntit yTransactionImpl.commit(DatastoreEntityTransaction Impl.java:55)
at org.springframework.orm.jpa.JpaTransactionManager. doCommit(JpaTransactionManager.java:467)

[hairinwind]

any body knows ?