与多个连接列的OneToMany关系

Question

I have this entity Friendship :

@ManyToOne
private User user1;
@ManyToOne
private User user2;
... and other properties of the friendship

Per friendship there is only one instance/record of this entity (not 2), so there can't be any issues where John is a friend of Jane but not the other way around. The entries are also normalized so the member of the friendship with the lowest ID is user1 , and the other member is user2 , meaning I can prevent duplicatie entries with a simple unique constraint.

To get the friendships of a certain user I perform a query with

WHERE user1 = :me OR user2 = :me .

Is it possible to map that onto a @OneToMany Set<Friendship> property of User ?

( Friendship has other properties, so this is not simply @ManyToMany Set<User> friends )

Answer 1

The short answer is no. For a bi-directional fetch, I believe the only way to do what you're asking is with a named query. Something along the lines of:

@Entity
@NamedQuery(name="User.friendships", query="
    select u 
    from User u
    where u in (
        select f.User1
        from Friendship f
        where f.User2.id = :userId
    ) or u in (
        select f.User2
        from Friendship f
        where f.user1.id = :userId
)")
public class User {

private Set<User> friends = new HashSet<User>()

public Collection<User> getFriendships(Session s) {
    return s.getNamedQuery("User.friendships")
        .setParameter("userId", this.id)
        .list();
}

}

The way I've done this in the past is to de-normalize the join table in some fashion, whether through duplicate columns (mapping an association in each direction), or duplicate rows. Either would allow you to simplify your fetch.

Answer 2

Yes it is possible. But why you need @OneToMany? You can use @ManyToMany. It will be same. In @ManyToMany annotation you can specify table where pairs of ids user1 - user2 will be stored. Then just do query to this table.