Hi there,

Could someone please explain what's going on here. I have a class named ScheduledCourse. It contains a collection of type Enroll. The Student class is the same. The Enroll class is an entity class (creates the ENROLL table) that serves as an intermediate join table between tables SCHEDULED_COURSES and STUDENTS. When I enroll a student I simply create an instance of type Enroll. Its constructor automatically adds the instance to the collections in ScheduledCourse and Student. When I run my test code for the first unit of work I load the collections. The println statements reveal the proper amounts. The instance of ScheduledCourse has three Enroll instances, while each of the Student instances have one.

So far, so good.

But wait, what's happening in the 2nd. unit of work? I use:
sc2 = em2.find(ScheduledCourse.class, index);
to bring the instance of ScheduledCourse back into the persistant context and use a println statement to have a look at the size of the collection of type Enroll. It's zero! This isn't good! Why is it zero?

See test code below:

//First unit of work
EntityManager em1 = emf.createEntityManager();
EntityTransaction tx1 = em1.getTransaction();
tx1.begin();

//Create a ScheduledCourse
ScheduledCourse sc1 = new ScheduledCourse(...);
em1.persist(sc1);
Long index = sc1.getId();
//Enroll several students into the course
//ScheduledCourse has a collection of type Enroll
//The following automatically adds these to the collections
//in ScheduledCourse and Student
Enroll enroll1 = new Enroll(company, admin1, student1, scheduled_course);
em1.persist(enroll1);
Enroll enroll2 = new Enroll(company, admin1, student2, scheduled_course);
em1.persist(enroll2);
Enroll enroll3 = new Enroll(company, admin1, student3, scheduled_course);
em1.persist(enroll3);

System.out.println("Adding students to the collections in scheduled course and students...");
System.out.println("scheduled_course.getEnrolled().size(): " + scheduled_course.getEnrolled().size()); //reveals 3
System.out.println("enroll1.getStudent().getEnrolled().size(): " + enroll1.getStudent().getEnrolled().size());//reveals 1
System.out.println("enroll2.getStudent().getEnrolled().size(): " + enroll2.getStudent().getEnrolled().size());//reveals 1
System.out.println("enroll3.getStudent().getEnrolled().size(): " + enroll3.getStudent().getEnrolled().size());//reveals 1

tx1.commit();
em1.close();

//Perform 2nd Unit of work
EntityManager em2 = emf.createEntityManager();
EntityTransaction tx2 = em2.getTransaction();
tx2.begin();

ScheduledCourse sc2 = em2.find(ScheduledCourse.class, index);

System.out.println("Enroll collection size: " + sc2.getEnrolled().size()); //reveals 0 Zero

tx2.commit();
em2.close();

The following code reveals that the records got recorded in the ENROLL table:

List enrolled = em2.createQuery("select e from Enroll e").getResultList();
System.out.println(enrolled.size() + " students enrolled"); //reveals 3

That's fine, but the collection is showing zero. Did something happen to the collection when the instance of ScheduledCourse became detached from the persistent context? I don't get it...

Please advise,

Alan