Question

I’ve got Parent – Child relationship OneToOne, but two of them. Annotations are not good but it produces good DB schema, yet code is not working. If I try to save Parent instance, Hibernate at first tries to save child1 and child2 – but it breaks FK defined in Child -> because owner doesn’t exist yet in DB…So I need to save Parent and then child1, and child2.

If I could do that it doesn’t help, because when I try to load Parent, Hibernate will not know which record in Child table belongs to child1 or child2…So in this case I would need to specify one condition in join for child1 something like “where type = 1” and for child2 “where type = 2″…

Just to clarify: in Child table there will be zero or one child for one Parent with ChildType.A (always child1) and zero or one child with ChildType.B (always child2).

I need to save xml which looks like this:

<parent id="" oid="">
   <child1 (and other attributes)>
   <child2 (and other attributes)>
<parent>

Both child1 and child2 elements are the same type therefore are type of Child in java classes. Only difference is element name (in java I differentiate them with ChildType). Only identification for children are id and oid attributes from parent. They points to another Parent hence target in Child.

I need to change annotations somehow to get this working…Do you guys have some ideas, because I’m really stuck???

Parent.java

public class Parent {

    private String oid
    private Long id;

    private Child child1;
    private Child child2;

    @Id
    @GeneratedValue(generator = "IdGenerator")
    @GenericGenerator(name = "IdGenerator", strategy = "com.example.IdGenerator")
    @Column(name = "id")
    public Long getId() {
        return id;
    }

    @Id
    @GeneratedValue(generator = "OidGenerator")
    @GenericGenerator(name = "OidGenerator", strategy = "com.example.OidGenerator")
    @Column(name = "oid", nullable = false, updatable = false, length = 36)
    public String getOid() {
        return oid;
    }

    @OneToOne(optional = true, fetch = FetchType.EAGER)
    @Cascade({org.hibernate.annotations.CascadeType.ALL})
    public Child getChild1() {
        return child1;
    }

    @OneToOne(optional = true, fetch = FetchType.EAGER)
    @Cascade({org.hibernate.annotations.CascadeType.ALL})
    public Child getChild2() {
        return child2;
    }
}

Child.java

public class Child {

    private Parent owner;
    private String ownerOid;
    private Long ownerId;
    private ChildType type;

    private Parent target;

    @MapsId("owner")
    @ManyToOne(fetch = FetchType.LAZY)
    @PrimaryKeyJoinColumns({
            @PrimaryKeyJoinColumn(name = "owner_oid", referencedColumnName = "oid"),
            @PrimaryKeyJoinColumn(name = "owner_id", referencedColumnName = "id")
    })
    public Parent getOwner() {
        return owner;
    }

    @MapsId("target")
    @ManyToOne(fetch = FetchType.LAZY)
    @PrimaryKeyJoinColumns({
            @PrimaryKeyJoinColumn(name = "target_oid", referencedColumnName = "oid"),
            @PrimaryKeyJoinColumn(name = "target_id", referencedColumnName = "id")
    })
    public Parent getTarget() {
        return target;
    }

    @Id
    @Column(name = "owner_id")
    public Long getOwnerId() {
        if (ownerId == null && owner != null) {
            ownerId = owner.getId();
        }
        return ownerId;
    }

    @Id
    @Column(name = "owner_oid", length = 36)
    public String getOwnerOid() {
        if (ownerOid == null && owner != null) {
            ownerOid = owner.getOid();
        }
        return ownerOid;
    }

    @Id
    @Column(name = "target_id")
    public Long getTargetId() {
        if (targetId == null && target != null) {
            targetId = target.getId();
        }
        return targetId;
    }

    @Id
    @Column(name = "target_oid", length = 36)
    public String getTargetOid() {
        if (targetOid == null && target != null) {
            targetOid = target.getOid();
        }
        if (targetOid == null) {
            targetOid = "";
        }
        return targetOid;
    }

    @Id
    @Enumerated(EnumType.ORDINAL)
    public ChildType getType() {
        if (type == null) {
            return ChildType.A;
        }
        return type;
    }
}

ChildType.java

public enum ChildType {
    A, B;
}

I also tried to use mappedBy approach mappedBy approach but there are still problems with loading – I can’t tell hibernate which child record belogs to which child class member variable.

Answer 1

there are too many things I don’t quite get in your solution to give a good answer but just some thoughts:

Consider using inheritance instead of ChildType enum. So you would have ChildA and ChildB extending Child.

That way you Parent can have:

private ChildA child1;
private ChildB child2;

Instead of having a composite primary key, I would consider using a unique auto generated key and then add a unique constraint on on the other id and oid fields. It should make the child parent relationships easier and you can have different parent implementation for ChildA and ChildB:

In ChildA:
@OneToOne(mappedBy="child1")
public Parent getParent() {
    return parent;
}

And

In ChildB:
@OneToOne(mappedBy="child2")
public Parent getParent() {
    return parent;
}

And in Child just:

public abstract Parent getParent();

Now the whole Parent/Owner/Target thing I still didn’t quite grasp.