JPA Inheritance issue

hibernate, inheritance, java, jpa

Solution

The problem is not caused by `@JoinColumn`. The reason is Lazy Loading. I manage to pinpoint your problem in simpler example. Forgive me for changing convention from Father to Parent.

In the example below, uninitialized Element is type of `jpa.inheritance.issue.Parent_$$_javassist_1`. It is a Hibernate Proxy - dynamically created subclass of Parent. You can "unproxy" it by invoking Hibernate proprietary API `getHibernateLazyInitializer().getImplementation()`.

Collection of `elementCollection` is also Lazy Initialized. The type of the collection is `org.hibernate.collection.PersistentBag` which is being initilized with correct data at the time of first access. Collection is initialized all at once. Please see the test which successfully passed green with your exact version of Hibernate (3.3.0.SP1/3.4.0.GA).

    @Test
    public void test() {
        Child c = new Child();
        em.persist(c);

        Another a = new Another();
        a.setElement(c);
        Collection<Parent> col = new ArrayList<Parent>();
        col.add(c);
        a.setElementCollection(col);
        em.persist(a);
        c.setAnother(a);

        long idx = a.getId();
        tx.commit();

        // I'm cleaning the cache to be sure that call to a.getElement() will return proxy.
        em.clear();
        tx = em.getTransaction();
        tx.begin();

        a = em.find(Another.class, idx);
        Assert.assertNotNull(a);
        Parent p = a.getElement();
        // At this point p is a type of jpa.inheritance.issue.Parent_$$_javassist_1

        Assert.assertTrue(p instanceof Parent);
        Assert.assertFalse(p instanceof Child);

        // At this point a.elements is a not initialized (empty) collection of type org.hibernate.collection.PersistentBag
        // When we access this collection for the first time, records are read from the database 
        Assert.assertEquals(1, a.getElementCollection().size());

        if (p instanceof HibernateProxy) {
            p =
                    (Parent) ((HibernateProxy) p).getHibernateLazyInitializer()
                            .getImplementation();
        }

        // At this point p is a type of jpa.inheritance.issue.Child
        Assert.assertTrue(p instanceof Child);
    }

    @Entity
    public class Another {

        @JoinColumn(name = "element_id", referencedColumnName = "id", nullable = false)
        @ManyToOne(fetch=FetchType.LAZY)
        private Parent element; 
        public Parent getElement() {
            return element;
        }

        public void setElement(Parent element) {
            this.element = element;
        }

        @OneToMany(cascade = CascadeType.ALL, mappedBy = "another", fetch = FetchType.LAZY)
        public Collection<Parent> elements;

        public Collection<Parent> getElementCollection() {
            return elements;
        }

        public void setElementCollection(Collection<Parent> elementCollection) {
            this.elements = elementCollection;
        }

        // @Id ...
    }

    @Entity
    @Inheritance(strategy = InheritanceType.JOINED)
    public class Parent {
        @ManyToOne
        private Another another;

        public Another getAnother() {
            return another;
        }

        public void setAnother(Another another) {
            this.another = another;
        }

        // @Id ...
    }

    @Entity
    public class Child extends Parent {         
    }

You don't need `@DiscriminatorColumn` nor `@DiscriminatorValue` because those annotations are needed with `InheritanceType.SINGLE_TABLE` as an only resort to determine the type. With `InheritanceType.JOINED` Hibernate is able to determine polymorphic type by checking if there is a record in both (Parent and Child) tables with the same Id. You can turn on hibernate logging to see how the query to determine the type looks like. It works like this:

select
    another0_.id as id0_1_,
    another0_.element_id as element2_0_1_,
    parent1_.id as id1_0_,
    parent1_1_.name as name2_0_,
    case
        when parent1_1_.id is not null then 1
        when parent1_.id is not null then 0
        else -1
    end as clazz_0_
from
    Another another0_
inner join
    Parent parent1_
        on another0_.element_id=parent1_.id
left outer join
    Child parent1_1_
        on parent1_.id=parent1_1_.id
where
    another0_.id=?

Problem

Original source