Hibernate bidirectional @ManyToOne, updating the not owning side not working

Question

I have a really simple setup to try out a bidirectional mapping with annotations:

@Entity
public class TypeA extends AbstractModel<TypeA> {

    @Id
    @GeneratedValue
    private int id;

    @OneToMany(mappedBy="a")
    private Collection<TypeB> bs;

    // Getters & Setters ... 
}

and

@Entity
public class TypeB extends AbstractModel<TypeB> {

    private static final long serialVersionUID = -3526384868922938604L;

    @Id
    @GeneratedValue
    private int id;

    @ManyToOne()
    @JoinColumn(name="a_id")
    private TypeA a;
}

When I set the property TypeA.bs this does not affect the mapping, although it should. See the following example:

TypeB b = new TypeB();
this.typeBDao.save(b);

TypeA a = new TypeA();
a.setBs(ListUtils.createList(b));

System.out.println(a.getBs()); // output: [TypeB@25fe4d40]

this.typeADao.save(a);

System.out.println(a.getBs()); // output: [TypeB@25fe4d40]

this.typeADao.refresh(a);

System.out.println(a.getBs()); // output: []

this.typeBDao.refresh(b);
System.out.println(b.getA()); // output: null

If the mapping is bidirectional, the collection should be populated and the property a of b should be updated, but it isnt. Any ideas?

Edit Thanks for your help folks, now I got it!

Answer 1

For a consistent domain model you should always set both sides of the relation, like this:

TypeB b = new TypeB();

TypeA a = new TypeA();
a.setBs(ListUtils.createList(b));
b.setA(a);   

this.typeBDao.save(b);
this.typeADao.save(a);

When your entities are in an inconsistent state, JPA will always store values according to the object state of the owning side of the JPA relation. In this case TypeB owns the relation to TypeA. Thus if an object of TypeB does not have a reference to TypeA, JPA assumes there is no relation defined.