Question

Probably a common problem, but anyway:

Let’s say I have a simple entity Task with a list of Activity entities:

@Entity
@Table(name = "task")
public class Task {

    @Id
    @Column(name = "id")
    private Integer id;

    @OneToMany
    @JoinColumn(name = "task_id")
    private List<Activity> activities;
}

And Activity entity is pretty simple:

@Entity
@Table(name = "activity")
public class Activity {

    @Id
    @Column(name = "id")
    private Integer id;

    @Column(name = "activity_type")
    private int type;
}

And here I want to do some magic — based on Activity type I want hibernate to stuff Task.activities with different Activity implementation, for example if Activity.type == 1, I want to have ActivityAImpl there, if Activity.type == 2 – it should be ActivityBImpl and so on.. Basically all my activities would implement a single interface with a some method “execute()”.

Hopefully there is a simple solution for that.

Thanks for your help!

Answer 1

Take a look at @MappedSuperClass and @Discriminator annotions. I guess that is what you are looking for. Here is an example.

@Entity
@Table(name = "activity)
@Inheritance
@DiscriminatorColumn(name = "activity_type")
public abstract class Activity {

    @Id
    @Column(name = "id")
    private Integer id;

    @Column(name = "activity_type")
    private int type;

    public abstract void execute();
}

@Entity
@DiscriminatorValue("1")
public class ActivityImplA extends Activity {

     public void execute() {
        .... Impl a
     }

}

@Entity
@DiscriminatorValue("2")
public class ActivityImplB extends Activity {

     public void execute() {
        .... Impl b
     }

}