Google App Engine ancestor query in JPA

Question

What is the way to implement ancestor query for the app engine in JPA?

I know about @Extension annotation but how is it used in the query?

   @Entity
   public class Event {
       @Id
       @GeneratedValue(strategy = GenerationType.IDENTITY)
       @Extension(vendorName="datanucleus", key="gae.parent-pk", value = "true")
       private Key id;
   }