Question

I have 3 tables: Project, User and Role. Now I would like to have a table Project2User2Role with the embedded key id_project, id_user,.

I tried it using @OneToMany relations in all three entities but I think I cannot build it like that.

And I also tried to build a Project2User2Role entity class by my self but than I have to make an idclass without the @ManyToOne relations.

How could the solution look like?

Answer 1

The reference manual says that you can embed a relationship inside an embedded ID :

While not supported in JPA, Hibernate
lets you place your association
directly in the embedded id component

You should thus define a Project2User2Role entity with an ID of type Project2User2RoleId:

@Entity
public class Project2User2Role {
    @EmbeddedId
    private Project2User2RoleId id;

    public User getUser() {
        return this.id.getUser();
    }

    public Project getProject() {
        return this.id.getProject();
    }

    // ...
}

The Project2User2RoleId class would look like this :

@Embeddable 
public class Project2User2RoleId {
    @ManyToOne(optional = false)
    @JoinColumn(name = "project_id")
    private Project project;

    @ManyToOne(optional = false)
    @JoinColumn(name = "user_id")
    private User user;

    @ManyToOne(optional = false)
    @JoinColumn(name = "role_id")
    private Role role;

    // constructor
    // getters
    // equals and hashCode
}