I hope it is okay to ask for help here :)

would like to ask for help on my hibernate project

I have 3 tables: student, class and student_class
student_class is a joint table for student and class

the following is the setup:

Student.java
@ManyToMany(
targetEntity=Class.class,
cascade={CascadeType.PERSIST, CascadeType.MERGE},
fetch=FetchType.EAGER
)
@JoinTable(
name="student_class",
joinColumns=@JoinColumn(name="student_id"),
inverseJoinColumns=@JoinColumn(name="class_id")
)
@Fetch(value = FetchMode.SUBSELECT)
public Set<Class> classes;

_____
Class.java
@ManyToMany(
cascade = {CascadeType.PERSIST, CascadeType.MERGE},
targetEntity = Student.class
)
@JoinTable(
name="student_class",
joinColumns=@JoinColumn(name="class_id"),
inverseJoinColumns=@JoinColumn(name="student_id")
)
public Set<Student> students;


What I want to do is when the student changes one of its classes the joint table (student_class) should be updated. But this is not happening in my current implementation.

for example
Student A has Math and Science classes. This creates 2 records in the joint table.
Now Student A decides to change Science class to French class. In my assumption, this should delete the A-and-Science record and inserts the A-and-French record in the joint table. But the A-and-Science record is not deleted that results to 3 classes for Student A.

How do I implement the correct mapping in order for A-and-Science to be deleted?

Thanks for the help.
rein