How to map OneToMany relationship from Superclass

Question

Hello! Recently I was stuck with such problem, and I hope the solution I provide below will help some other JPA newbies like me. If there is better solution please post it here!

The problem is as follows:

I want to create OneToMany relationship from classes Book and CD to class Tag. In order to unify all the logic regarding class Tag from Book and CD I create @MappedSuperclass class Item, and make Book and CD descendants of class Item. But when I try to map List <Tag> tags with @OneToMany” in that superclass I get nothing good..

Answer 1

My solution:

In order to do an ORM mapping one should understand first what he actually wants to see in the database. So when I realized that reasonable solution is to create several transition tables between descendants of Item and Tag, I understood, that this may be accomplished using @ManyToMany. And it works fine!
Listing below.

@MappedSuperclass
public class Item extends Model {
    @Id
    public  Long id;
    @ManyToMany(cascade = CascadeType.ALL)
    public List<Tag> tags;
    public String name;
    <…> }    
@Entity
public class Book extends Item {
    public int pageNum;
    <…> }
@Entity
public class CD extends Item{
    public int size;
    <…> }
@Entity
public class Tag extends Model{
    @Id
    public Long id;
    public String text;
    <…> }

PS I'd post also class and er diagrams, but currently I got no r8n to post images.