Inheritance strategy in Hibernate: making an entity abstract and replacing with sub classes

Question

I have currently existing database and hibernate mapping for it. There is a central table and corresponding entity (PersistentObject). Many of the other tables and entities refer to PersistentObject via @ManyToOne or @OneToOne mapping.

Now I'd like to make current PersistentObject abstract and introduce two sub classes Sub1Object and Sub2Object that inherit from PeristentObject.

I can't change the table where PersistentObject is stored, otherwise adding new tables etc is possible. The table contains enough information so that I can use e.g @DiscriminatorColumn to separate Sub1Object and Sub2Object rows if needed.

Is it possible make this change so that my current HQL queries still work without modifications? I have lot of queries like from PersistentObject where foo = ? and from OtherEntity other where other.persistentObject = ?. I'd like that those queries start returning concrete subclasses instead of the super class. I don't want to code separate queries for Sub1Object and Sub2Object.

Is this at all possible? What inheritance strategy I should use?

Answer 1

You must use table-per-class-hierarchy strategy, because you want to use discriminator column :) As for me, hibernate should work with what you described without any problem. Mapping subclasses as hiberante-aware classes and providing proper discriminator should be enough.

Just be warning, that sometimes instanceof directive will not work, because you will get proxy object, but you can use Hibernate.getClass(Obj...) to check.

Answer 2

I can't change the table where PersistentObject is stored, otherwise adding new tables etc is possible. The table contains enough information so that I can use e.g @DiscriminatorColumn to separate Sub1Object and Sub2Object rows if needed.

You CAN use a JOINED inheritance strategy (with or without a discriminator). For the top class:

@Entity
@Inheritance(strategy = InheritanceType.JOINED)
@DiscriminatorColumn(name = "DISC")
public abstract PersistentObject {

    @Id @GeneratedValue
    private Long id;

    ...
}

And for example:

@Entity
public class Sub1Object extends PersistentObject {
    ...
}

This won't change the existing table.

Is it possible to make this change so that my current HQL queries still work without modifications? I have lot of queries like from PersistentObject where foo = ? and from OtherEntity other where other.persistentObject = ?. I'd like that those queries start returning concrete subclasses instead of the super class.

They will work and will return instances of all persistent classes that extend that class.