PersistenceException：传递给持久化的分离实体

Question

I read through a lot of threads on stackoverflow about the same exception but I didn't find anything that actually helps me so here I am:

I have got 2 entity classes:

Teacher class:

@Entity
@Table(name="teachers")
public class Teacher extends Model{

public Teacher()

@Id
@GeneratedValue(strategy=GenerationType.IDENTITY)
private int id;

@OneToMany(fetch=FetchType.LAZY, mappedBy="teacher")
private List<Student> students;

//getter and setter
}

Student class:

@Entity
@Table(name="students")
public class Student extends Model{

public Student(){}
public Student(Teacher teacher){this.teacher = teacher;}

@Id
@GeneratedValue(strategy=GenerationType.IDENTITY)
private int id;

@ManyToOne(fetch=FetchType.LAZY, cascade={CascadeType.ALL})
@JoinColumn(name="teacher_id")
private Teacher teacher;

//getter and setter
}

Model class:

public abstract class Model implements Serializable {

public void save() {
EntityManager entityManager = HibernateUtil.getEntityManagerFactory().createEntityManager();
entityManager.getTransaction().begin();
entityManager.persist(this);
entityManager.getTransaction().commit();
entityManager.close();
}

public void merge(){
EntityManager entityManager = HibernateUtil.getEntityManagerFactory().createEntityManager();
entityManager.getTransaction().begin();
entityManager.merge(this);
entityManager.getTransaction().commit();
entityManager.close();
}
}

I created a Test class to insert some values into the db:

public class TestClass{

Teacher teacher;
teacher = new Teacher();

Student student_1;
Student student_2;

student_1 = new Student(teacher);
student_1.save();

student_2 = new Student(teacher);
student_2.save();

}

student_2.save() throws the exception (teacher is a detached entity). But why? I understand, that teacher is already persisted and the transaction is over but CascadeType.ALL includes merge aswell, which should take care of it (or obviously not..i might be wrong here) but the following then works:

student_2.merge() instead of student_2.save() works though~

Another solution would be to remove the cascadeType.ALL and persist teacher before persisting student but that's not what I want.

Answer 1

Cascade works different from what you think. It calls the same function on the related objects. So if you call em.persist(student) it will call persist on teacher.

I think you probably don't want to cascade anything from the student to the teacher. If you persist the teacher in a transaction and then only link it to the students in the other transactions you don't have cascade anything to the teacher.