Question

I have this 2 classes:

@Entity
public class Student extends User {
    ...
    @ManyToMany(mappedBy = "members", fetch = FetchType.EAGER)
    private Set<Group> groups;
    ...
    public void addGroup(Group group) {
        groups.add(group);
    }
}


@Entity
@Table(name = "Group")
public class Group implements Serializable {
    ...    
    @ManyToMany(fetch = FetchType.EAGER)
    @JoinTable(name = "member_of")
    private Set<Student> members;
    ...    
    public void addMember(Student student) {
        members.add(student);
    }
}

This is the session bean which manages their relationships:

@Stateless
public class StudentManager extends AcademicManager implements StudentManagerRemote {
    ...
    @Override
    public void requestAnswer(long requestID, RequestAnswer answer) {
        Request request = entityManager.find(Request.class, requestID);
        if(answer == RequestAnswer.YES) {
            Student student = request.getStudent();
            Group group = request.getGroup();
            group.addMember(student);
            student.addGroup(group);
            entityManager.flush();
            entityManager.clear();
        }
    }
    ...
}

It does not update db. Why? I do the same for other relationship, but this is the only ManyToMany I have.
I also tried with a query, but did not work.

EDIT: I edited the code like Mr.J4mes suggested, but it still does not work.
Moreover: why are .flush() and .clear() not necessary?

Answer 1

When you’re calling group.getMembers(), you are just getting a copy of the Set that contains Student from the Group entity. Hence, when you update it, the Set inside your Group entity does not get updated. You have 2 choices:

1. Get the Set from Group and set back after updating it:

   Set<Student> members = group.getMembers();
   members.add(student);
   group.setMembers(members);
   

2. Add a method addStudent inside the Group entity & a method addGroup inside the Student entity to call in your requestAnswer method. It would be something like this:

   @Entity
   public class Group implements Serializable {
       private Set<Student> members;
   
       public void addMember(Student student) {
          this.members.add(student);
       }
   }
   
   @Stateless
   public class StudentManager extends AcademicManager implements StudentManagerRemote {
       @PersistenceContext
       private EntityManager em;
       ...
       @Override
       public void requestAnswer(long requestID, RequestAnswer answer) {
           Request request = entityManager.find(Request.class, requestID);
           if(answer == RequestAnswer.YES) {
               Student student = request.getStudent();
               Group group = request.getGroup();
               group.addMember(student);
               student.addGroup(group);
           }
       }
       ...
   }
   

Besides, you don’t really need to call em.flush() and em.cancel().