Google App Engine ancestor query in JPA

Google App Engine ancestor query in JPA

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;
}