直接透過 Entity 作 remove 再 save 進 DB 是沒有用的，要用 EntityManager 來 remove

//沒用，存進 DB 也不會刪除
public void removeStudent(Student stu){
    this.students.remove(stu);
}

要這樣才行

entityManager.remove( child );

其實 Hibernate 同樣也是要用 session 刪除

However, the following code:

Parent p = (Parent) session.load(Parent.class, pid);
Child c = (Child) p.getChildren().iterator().next();
p.getChildren().remove(c);
c.setParent(null);
session.flush();

will not remove c from the database. In this case, it will only remove the link to p and cause a NOT NULLconstraint violation. You need to explicitly delete() the Child.

Parent p = (Parent) session.load(Parent.class, pid);
Child c = (Child) p.getChildren().iterator().next();
p.getChildren().remove(c);
session.delete(c);
session.flush();

資料來源

• Enterprise JavaBeans - JPA: How to delete children in a OneToMany relationship?
• Hibernate Doc - 21.3. Cascading life cycle