javahibernatejpaclass-hierarchy

Retrieve only the superclass from a class hierarchy

I have an scenario as the following:

@Entity
@Table(name = "ANIMAL")
@Inheritance(strategy = InheritanceType.JOINED)

public class Animal implements Serializable {

    @Id
    @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "S_ANIMAL")
    @SequenceGenerator(name = "S_ANIMAL", sequenceName = "S_ANIMAL", allocationSize = 1)
    public int getNumero() {
        return numero;
    }

    public void setNumero(int numero) {
        this.numero = numero;
    }
        .
        .
        .
}

and as the subclass:

@Entity
@Table(name = "DOG")
public class Dog extends Animal {

    private static final long serialVersionUID = -7341592543130659641L;
        .
        .
        .
}

I have a JPA Select statement like this:

SELECT a FROM Animal a;

I'm using Hibernate 3.3.1

As I can see the framework retrieves instances of Animal and also of Dog using a left outer join.

Is there a way to Select only the "part" Animal? I mean, the previous Select will get all the Animals, those that are only Animals but not Dogs and those that are Dogs.

I want them all, but in the case of Dogs I want to only retrieve the "Animal part" of them.

I found the @org.hibernate.annotations.Entity(polymorphism = PolymorphismType.EXPLICIT) but as I could see this only works if Animal isn't an @Entity.

Thanks a lot.

Solution

  • Short answer: The behaviour you describe complies with JPA standard. There is no JPA standard way to restrict the JPA provider to retrieve only the superclass.

    The provider can choose query implementation logic to achieve functionality, consistency & performance. As long as it respects your entity annotations and returns the requested information in the query all is good. Think of the outer join to Dog as private implementation detail that should not concern you. The provider has coded an outer join to assist performance and consistency.

    Consider: