JPA many to many - do i have to remove\add from both collection sets? - spring

Question

I have a User entity,
And a Department Entity
i want to have #manyToMany relationship between them :
many users can have many departments.
in my User entity:
#ManyToMany(fetch = FetchType.LAZY)
#JoinTable(name = "UserDepartments", joinColumns = { #JoinColumn(name = "user_id", referencedColumnName = "id") }, inverseJoinColumns = { #JoinColumn(name = "department_id", referencedColumnName = "id") })
private Set<Department> departments;
and my Department entity has a SET as well of users..
my question is:
if i need to implement the method :
public void removeUserFromDepartment(User user, Department department) {
//bla bla
}
do i have to call
department.getUserCollection.remove(user);
AND
user.getDepartmentCollection.remove(department);
Or is there a way to maintain this logic by only removing one of them ?
If i have to save both its pretty hard to maintain especially for someone who doesn't know about the many to many relation of the two entities..

Answer 1

When a OneToMany or ManyToMany relationship exists in JPA the client code is responsible for managing the relationship. This means that you must explicitly remove the object from both sides of the relationship.
So lets say you have a User instance and need to remove a department.
User user = dao.findUser(1L); //Find a user, psuedo code
Department dept = user.getDepartments().get(0);
dept.getUsers().remove(user);
user.getDepartments().remove(user);
dao.update(user); //once again psuedo code
To use the code above you should add a cascade type to the relationship in the user entity:
#ManyToMany(fetch = FetchType.LAZY, cascade=CascadeType.ALL)
#JoinTable(name = "UserDepartments", joinColumns = { #JoinColumn(name = "user_id", referencedColumnName = "id") }, inverseJoinColumns = { #JoinColumn(name = "department_id", referencedColumnName = "id") })
private Set<Department> departments;
This will cause saves on the User entity to be cascaded to the Departments entity. Just a reminder save is psuedo code, it will boil down to a call on the EntityManager such as persist or merge.