MappedSuperClass

Question

Hi *,
I'm having some trouble with a MappedSuperClass I defined. Let me build up...
Node, which keeps a reference to a Reading object:
@Entity
public class Node implements Serializable {
    private static final long serialVersionUID = 1L;
    private Long id;
    @EmbeddedId
    private NodePK nodePK;
    @ManyToOne(cascade={CascadeType.PERSIST, CascadeType.REMOVE})
    private Network network;
    @OneToOne
    private Reading latestReading;
    ...//(constructors and methods)
}Unit, which is the superclass of Reading:
@MappedSuperclass
public abstract class Unit implements Serializable {
    private static final long serialVersionUID = 1L;
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;
    @OneToOne
    private Node node;
    public Unit() {}
    ...//(constructors and methods)
}Reading, which inherits from Unit
@Entity
public class Reading extends Unit implements Serializable {
    //private Node node;                         <- PROBLEM
    public Reading() {}
    ...//(constructor and methods)
}So, I create a Node object which stores its latest Reading-object, which is a subclass of Unit. To make Unit a MappedSuperClass, I annotated the class accordingly and extend its subclass Reading.
I further have a stateless session bean which does the following:
Node n1 = new Node(1, net1);
em.persist(n1);
Reading reading1 = new Reading(n1, null, 1, 2, 3);
em.persist(reading1);
n1.setLatestReading(reading1);
removeNode(1);
private void removeNode(int address) {
        Node node = (Node) em.createQuery("SELECT x " +
                "FROM Node x " +
                "WHERE x.nodePK.nodeAddress = :addr").setParameter("addr", address).getSingleResult();
        em.remove(node);
}So basically just creating a node, creating a reading, set this reading as the nodes latest reading and finally remove this all from persistence. And there it goes wrong! When running my app I get:
Caused by: org.apache.derby.client.am.SqlException: DELETE on table 'NODE' caused a violation of foreign key constraint 'READINGNODEADDRESS' for key (1,321).  The statement has been rolled back.
...I tried a bunch off stuff including adding AssociationOverride statements and moving the node property from the superclass to its subclasses. The latter works, but is kind of foolish because then the superclass has no other significant properties and I could as well not use a superclass (FYI, Reading will not be the only class extending Unit).
Something that also seems to work, is adding the node attribute both to the superclass and the subclass (so uncommenting the problem line I indicated above).
Can someone help me with this? I've been struggling for days with this and about to give up so...
Thanks!
Vaak

Answer 1

I am currently using TopLink Essential build 17. I was able to take my standard Employee demo model and refactor a BaseEntity class out of the Employee class.
BaseEntity
@MappedSuperclass
@TableGenerator(name = "emp-seq-table", table = "SEQUENCE",
                pkColumnName = "SEQ_NAME", valueColumnName = "SEQ_COUNT",
                pkColumnValue = "EMP_SEQ", allocationSize=50)
public abstract class BaseEntity {
    @Id
    @GeneratedValue(strategy = GenerationType.TABLE, generator = "emp-seq-table")
    private int id;
    @Version
    private long version;
    public BaseEntity() {
    public void setId(int id) {
        this.id = id;
    public int getId() {
        return id;
    public void setVersion(long version) {
        this.version = version;
    public long getVersion() {
        return version;
}The Employee class now looks basically like:
@Entity
@AttributeOverride(name="id", column=@Column(name="EMP_ID"))
public class Employee extends BaseEntity implements Serializable {Doug