JPA 2.1: Treat

JPA 2.1 introduces a new keyword treat in JPQL which allow path expressions to be treated as a subclass, giving access to subclass specific state.

Create two specified Comments, VoteUp and VoteDown.

@Entity
public class VoteUp  extends Comment{

    public VoteUp() {
        super("Up");
    }
    
}

@Entity
public class VoteDown  extends Comment{

    public VoteDown() {
        super("Down");
    }
    
}

Add an @Inheritance annotation on Comment entity.

@Entity
@Inheritance(strategy = InheritanceType.JOINED)
public class Comment implements Serializable {
}

In the ViewPostBean, fetch the counts of voteUp comments and voteDown comments.

private void fetchVoteDown() {
 this.voteDown = ((Long) (em.createQuery("select count(vu)from Post p join treat(p.comments  as VoteDown) vu where p.id=:id")
  .setParameter("id", this.id)
  .getSingleResult())).intValue();
}

private void fetchVoteUp() {
 this.voteUp = ((Long) (em.createQuery("select count(vu)from Post p join treat(p.comments  as VoteUp) vu where p.id=:id")
  .setParameter("id", this.id)
  .getSingleResult())).intValue();
}

Also create a voteUp() and voteDown() methods to create VoteUp and VoteDown objects.

public void voteUp() {
 final VoteUp comment = new VoteUp();
 comment.setPost(this.post);
 this.post.getComments().add(comment);

 em.merge(this.post);
 em.flush();

 fetchVoteUp();
}

public void voteDown() {
 final VoteDown comment = new VoteDown();
 comment.setPost(this.post);
 this.post.getComments().add(comment);

 em.merge(this.post);
 em.flush();

 fetchVoteDown();
}

The sample codes are hosted on my github.com account, check out and play it yourself.

https://github.com/hantsy/ee7-sandbox