JPA @OneToMany结果为空

Question

I'm trying acces a MySQL databese through JPA, but the result is always empty.

public class Part {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private long id;
    ...
    @ManyToOne(fetch = FetchType.EAGER)
    @JoinColumn(name = "owner", referencedColumnName = "id")
    private Owner owner;
    ...
}

public class Owner {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private long id;
    ...
    @OneToMany(mappedBy = "owner")
    private Set<Part> parts;
    ... 
}

Whenever I try to get the parts connected to an owner, the resulting Set is empty. After calling eg. size() method, it's still empty.

List<Owner> owners = em.createQuery("...", Owner.class).getResultList();

for (Owner o : owners) {
    System.out.print(o + ": ");
    Set<Part> parts = o.getParts();
    parts.size()
}

Here the parts Set is always empty. In the database it seems to be connected, but I can't get any results out of it. What am I doing wrong?

Answer 1

My guess is that you are not adding to both sides of your relationship when creating your objects.

See, http://en.wikibooks.org/wiki/Java_Persistence/Relationships#Object_corruption.2C_one_side_of_the_relationship_is_not_updated_after_updating_the_other_side