Inheritance Mapping: How do I do this?

drew_k · **Posted:** Mon Oct 20, 2008 6:56 pm

Hibernate version:
3.4 using EntityManager

Mapping documents:
N/A

Code between sessionFactory.openSession() and session.close():
N/A, using Seam Managed Persistence

Full stack trace of any exception that occurs:
None

Name and version of the database you are using:
MySQL 5.0

The generated SQL (show_sql=true):
N/A

Debug level Hibernate log excerpt:
N/A

Problems with Session and transaction handling?
No

Code:

@Entity
@Table(name = "pet")
@Inheritance(strategy = InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(name = "type", discriminatorType = DiscriminatorType.INTEGER)
public abstract class Pet {
....
}

Code:

@Entity
@DiscriminatorValue("1")
public class Dog extends Pet {
....
}

Code:

@Entity
@DiscriminatorValue("2")
public class Cat extends Pet {
...
}

Code:

@Entity
@Table(name="human")
public class Human {

  @ManyToOne(fetch = FetchType.LAZY, optional = false)
  @JoinColumn(name = "pet_id", nullable = false)
  private Pet pet;
...
}

Code:

Human john = new Human();
john.setPet(new Cat());

Human mary = new Human();
mary.setPet(new Dog());

If I persist john and mary objects and retreive them from the database, Hibernate always returns Pet instances when I call john.getPet() or mary.getPet() and not Dog or Cat instances . What should I do to actually get the Dog/Cat instances? Is that even possible?

Thanks!

pbdavey · **Joined:** Thu May 22, 2008 9:48 am **Posts:** 13

Check instanceof for the returned objects from your getPet method. Of course it will return a Pet as that is what the method declares (I assume) in the abstract superclass. I'd imagine it can be cast into a subclass if you need to operate on it as a Dog/Cat. I'm not 100% about this, but it's my hunch.

drew_k · **Posted:** Mon Oct 20, 2008 9:14 pm

I already tried using instanceof/casting but the object that comes back from Hibernate is of type Pet (enhanced with JavaAssist/CGLIB) and cannot be cast to Dog or Cat.

nordborg · **Posted:** Tue Oct 21, 2008 2:18 am

This is expected when you use lazy loading as described by http://www.hibernate.org/hib_docs/v3/re ... ng-proxies

drew_k · **Posted:** Tue Oct 21, 2008 2:27 am

I've already read the documentation. In the doc, they suggest to define new interfaces as a workaround and they only show standard Hibernate configuration. I just don't know how it would apply to JPA/EntityManager annotations.

Thanks!