JPA：建模m：n关系和联接表值

Question

having am:n real-world relationship between some entities, eg user <---> group Now I want to model this relationship and store additional information based on it, eg a field "quality".

I heard that I would have to create a new join table user_group as follows:

id | user_ref | group_ref | quality
----------------------------------
1      1           1         0.5
2      1           2         1.3
...    ...         ...        ...

The corresponding entity has two related entities (private members) user and group, annotated with the @ManyToOne -annotation. On the other hand, both, my user and my group have a set of related user_group-entities, both private members and declared with the @OneToMany -annotation.

I have three questions:

1. Is this the right way to model the problem of additional fields in JPA 2.0?
2. I not allowed to use both user and group in user_group as primary key since they are not valid primary key types. Is it really necessary to declare a new primary.
3. Is this a common workflow with these join-tables/entities?

...

EntityManager em = ...
...
em.getTransaction().begin();
User u = new User("Pete");
Group g = new Group("Anonymous workaholics")
UserGroup ug = new UserGroup();
ug.addUser(u);
ug.addGroup(g);

em.persist(u); em.persist(g); em.persist(ug);
em.getTransaction().commit();
em.close()

Thanks a lot!

Answer 1

1. Yes, it's the good way to do it.
2. It's possible to have a composite primary key consisting of two ManyToOne associations, but it's much more complex, both in the mapping and in the rest of the application, to handle that, and it's also less efficient. You have an entity, so just do like for all the other entities and use an auto-generated, single-column primary key. The fact that this entity used to be a join table of a many-to-many association is irrelevant.
3. Yes, that seems OK, except the addUser() and addGroup() methods should be named setUser() and setGroup() : there's only one user and one group for a given UserGroup . I would also use another name for the entity itself. Something like "Membership" for example.