PersistenceException：已分离的实体传递到Persister

Question

我在stackoverflow上读了很多关于相同异常的文章，但没有找到任何实际帮助我的东西，所以我在这里：

@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
}

@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
}

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();
}
}

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();

}

Answer 1

Cascade的工作方式与你想象的不同。它在相关对象上调用相同的函数。因此，如果调用em.persist（student），它将调用persist on teacher。

我想你可能不想把任何东西从学生身上层层递送到老师身上。如果您在一个事务中持久化教师，然后只将其链接到其他事务中的学生，那么您就没有向教师级联任何内容。