Abstract Entity classes with Collections

Question

I trying to figure out a way to have a generics collection in a parent collection that can hold various entity types of the same super-class.
One thing I've tried was the following:
@Entity
public class ParentEnt implements Serializable {
    private static final long serialVersionUID = 1L;
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;
    @Column
    private String parentName;
    @OneToMany(mappedBy="parentEnt")
    private Collection<ChildAbs> children;
    /* Methods removed */
@Entity
public abstract class ChildAbs implements java.io.Serializable {
    protected static final long serialVersionUID = 1L;
    @Id
    protected Long id;
    @ManyToOne
    protected ParentEnt parentEnt;
    /* Methods removed */
@Entity
public class ChildEnt extends ChildAbs implements Serializable {
    @Column
    protected String childName;
}but this is the error I get:
Exception [TOPLINK-30005] (Oracle TopLink Essentials - 2.0 (Build b58g-fcs (09/07/2007))): oracle.toplink.essentials.exceptions.PersistenceUnitLoadingException
Exception Description: An exception was thrown while searching for persistence archives with ClassLoader: sun.misc.Launcher$AppClassLoader@11b86e7
Internal Exception: javax.persistence.PersistenceException: Exception [TOPLINK-28018] (Oracle TopLink Essentials - 2.0 (Build b58g-fcs (09/07/2007))): oracle.toplink.essentials.exceptions.EntityManagerSetupException
Exception Description: predeploy for PersistenceUnit [testPU] failed.
Internal Exception: Exception [TOPLINK-7250] (Oracle TopLink Essentials - 2.0 (Build b58g-fcs (09/07/2007))): oracle.toplink.essentials.exceptions.ValidationException
Exception Description: [class test.ParentEnt] uses a non-entity [class test.ChildAbs] as target entity in the relationship attribute [private java.util.Collection test.ParentEnt.children].So, it looks like there is no way to have a generics collection for an abstract entity class. Is there another way to accomplish what I want using a generics collection?
Additionally, entity ChildEnt generates an error due to the abstract class' id property not carrying forward (see Abstract Entities: http://java.sun.com/javaee/5/docs/tutorial/doc/bnbqa.html#bnbqo)
Exception Description: Entity class [class testprintservices.ChildEnt] has no primary key specified. It should define either an @Id, @EmbeddedId or an @IdClass.Any input would be appreciated.

Answer 1

As far as my first question, I was trying to make my collection so it can hold various type of entity classes, but now that I think about it, it was a bad idea. For instance, being able to hold ChildEnt and ChildEnt2 in the same collection. If JPA did allow this, it might be possible to save the entities to the DB, but the reverse wouldn't be possible.
Ok, regarding my 2nd question: I don't understand why having it as @Entity wouldn't. The links (mine and yours) clearly shows that @Id is carried forward in the example they provide. Anyway, since I can't use the superclass in the collection, this is pointless. I'll use the suggested annotation.
Thanks.