JPA polymorphic oneToMany

Question

I couldn\'t figure out how to cleanly do a tag cloud with JPA where each db entity can have many tags.

E.g

Post can have 0 or more Tags User can have 0 or mo

Answer 1

If you don't need polymorphic queries such as "get everything tagged tagged Foo", then you can also introduce a new entity (say TaggingTarget), and create a unidirectional one-to-one relation from User (Post, etc) to TaggingTarget and many-to-many relation between TaggingTarget and Tag:

@Entity
public class User {
    @OneToOne
    private TaggingTarget target;
    ...
}

@Entity
public class TaggingTarget {
    @ManyToMany(...)
    private Set tags;
    ...
}

@Entity
public class Tag {
    @ManyToMany(...)
    private Set targets;
    ...
}

The difference from Affe's solution is that you don't need a tag.getTaggedUsers(), tag.getTaggedPosts(), etc and can add new tagged entities without changing Tag. Tagged entities can be queried using JPQL:

select u from User u where :tag member of u.target.tags

or

select u from User u join u.target.tags t where t.name = :name