How can I get information out of a junction table using JPA/EclipseLink?

Answer 1

Try adding @ManyToMany(cascade = CascadeType.ALL) to your Cat object, to cascade your read operations.

With cascading set up properly you can change your saving code to;

Entitymanager em = emf.createEntityManager();

em.getTransaction().begin(); 
Student s = new Student(); 
Student s2 = new Student();
Cat c = new Cat();
// this should be c.setStudents(s2)
c.getStudents().add(s2); 
em.persist(c); 
em.getTransaction().commit();

Honestly don't fully understand why you are trying to assign two student sets into cats, you should join both sets into one and then add them to cats.

You also may want to look into explicitly setting your fetch type, @ManyToMany(fetch = FetchType.LAZY) or FetchType.EAGER.

Answer 2

Perhaps a better example would have been:

em.getTransaction().begin(); 
Student s = new Student(); 
Student s2 = new Student(); 
Cat c = new Cat(); 
em.persist(s); 
em.persist(s2);
em.persist(c); 
c.getStudents().add(s); 
c.getStudents().add(s2); 
em.getTransaction().commit(); 

and then trying to say

Cat c2 = em.find(Cat.class,c.getYID());
Student s3 = em.find(Student.class,s.getXID());
System.out.println(c2.getYID() + ": " + c2.getStudents());
System.out.println(s3.getXID() + ": " + s3.getCats());

The above prints:

1: {[[Movie: movieID=2], [Movie: movieID=3]]}

2: {[]}

This makes no sense to me, since the cat with id=1 has a movie in its set with id=2, but the movie with id=2 does not have a cat in its set with id=1. (Changing the cascade type didn't seem to affect the results.)

Thanks so much for replying.

B.J.