Add more object into an existing list in hibernate one to many unidirection

Question

I have looked into several example on hibernate one-to-many unidirectonal relation, and wonder that how can I add a new object into an existing list.

Assume that I have a class MyClass

@OneToMany(fetch=FetchType.LAZY, cascade={
                    CascadeType.PERSIST, 
                    CascadeType.REMOVE})
@org.hibernate.annotations.Cascade(org.hibernate.annotations.CascadeType.SAVE_UPDATE)
@JoinTable(name="enrolled_student", 
        joinColumns = {@JoinColumn(name="Class_ID")},
        inverseJoinColumns = {@JoinColumn(name="STUDENT_ID") })
public List<Student> getStudentList() { return studentList;}
public void setStudentList(List<Student> studentList) { this.studentList = studentList;}
private List<Student> List;

If I have a list of students such as s1, s2, s3. And now I want to to add student s4 into the class. How can I do that?

I have tried to do as:

@Override
@Transactional
public void addStudentToClass(MyClass class, Student student) {
    List<student> list = new ArrayList<Student>();
    list.add(student);
    budget.setStudentList(list);
    classDao.update(budget);
}

But it doesnt work as I expected. It erased all the existing student s1, s2, s3, then persist s4.

Answer 1

1. If myClass is detached but the studentList was already intialized:

   Override
   @Transactional
   public void addStudentToClass(MyClass myClass, Student student) {
       myClass.getStudentList().add(student);
       classDao.update(myClass);
   }
   

2. If myClass is detached but the studentList has not been previously initialized:

   Override
   @Transactional
   public void addStudentToClass(MyClass myClass, Student student) {
       MyClass attachedMyClass = classDao.merge(myClass);
       attachedMyClass.getStudentList().add(student);
   }