OneToMany JoinTable with a non Primary Key value

Question

I'm trying to replicate Facebook's wall in Java EE.

I've got 3 tables: User, Post, Wall.

User has an username(PK) and other fields, Post has an id(PK), an author(FK on User) and some other. Wall is a Join Table between Post and User.

The point is that the project compiles only if I have, in Wall, both post and user as PK. Having Post has PK and User just as a non null value won't work !

But having both of them as PK leads to a @ManyToMany, which means that the same post can be on many Walls, which is incorrect !

I've tried having just Post as PK in Wall and using this mapping :

Post:

@JoinTable(name = "Wall", joinColumns = {
    @JoinColumn(name = "id", referencedColumnName = "id")}, inverseJoinColumns = {
    @JoinColumn(name = "user", referencedColumnName = "username")})
@OneToMany
private User user;

User:

@ManyToOne
private Collection<Post> onWall;

but it says :

The target entity of the relationship attribute [user] on the class [class entity.Post] cannot be determined. When not using generics, ensure the target entity is defined on the relationship mapping..

What's wrong with my mapping?

Answer 1

How about two relationships User-to-Post, one for wall, one for authorship:

• User one-to-many to Post (posts stuck to user's wall, no join table),
• User one-to-many to Post (post's author),

with reverse relationships as needed.

For example:

@Entity
public class User {
    @OneToMany(mappedBy="onWall")
    private Collection<Post> wall;

    // You can skip this relationship, if you don't want it
    @OneToMany(mappedBy="author")
    private Collection<Post> authoredPosts;
}

@Entity
public class Post {
    @ManyToOne
    @JoinColumn(name="wallOwnerId")
    private User onWall;

    @ManyToOne
    @JoinColumn(name="authorId")
    private User author;
}