Question

好的，所以我想在一个表中映射组和学生，但我有一个错误：“对象引用未保存的瞬态实例”。如果我向所有人添加级联，它会说这个对象arleady存在。有什么问题？

以下是实体：

 @Entity  
 @Table(name = "listaosob") 
 public class listaOsob implements Serializable {

@Id
@GeneratedValue
private int id;
private static final long serialVersionUID = 1L;
@ManyToOne
private Student student;
@ManyToOne
private Grupa grupa;

@Entity  
@Table(name = "student") 
public class Student implements Serializable {

private static final long serialVersionUID = 1L;
@Id
@GeneratedValue
private int id;
private String imie;
private String nazwisko;
private String login;
private String haslo;
@Column(unique=true)
private int nr_albumu;
@Column(unique=true)
private String kod_karty;

 @Entity  
 @Table(name = "grupa") 
 public class Grupa implements Serializable {

private static final long serialVersionUID = 1L;
@Id
@GeneratedValue
private int id;
private String nazwa;
private Time godzina;
private String sala;
private String tydzien;
@ManyToOne
private Prowadzacy prowadzacy;

ejb代码：

 @Stateless
 @LocalBean
 public class StudentBean implements StudentBeanRemote, StudentBeanLocal {

 @PersistenceContext  
 private EntityManager eM; 

public StudentBean() {}

public void dodajStudenta(Student s) {
    eM.persist(s);
    eM.flush();
}

public void usunStudenta(Student s) {
    eM.remove(s);  
    eM.flush();
}

@SuppressWarnings("unchecked")
public ArrayList<Student> listaStudentow() {
    Query query = eM.createQuery("SELECT e FROM student e");
    return (ArrayList<Student>) query.getResultList();
}

@SuppressWarnings("unchecked")
public ArrayList<Student> listaStudentow(Grupa grupa) {
    Query query = eM.createQuery("SELECT e FROM listaosob e WHERE e.grupa = :grupa");
    query.setParameter("grupa", grupa);
    return (ArrayList<Student>) query.getResultList();
}

public void dodajGrupe(Grupa g) {
    eM.persist(g);
}

public void usunGrupe(Grupa g) {
    eM.remove(g);  
    eM.flush();
}

public void dodajListeOsob(listaOsob l) {
    eM.persist(l);
}

和示例测试：

Student name1 = new Student("new"+gen.nextInt(),"nazwisko","log", "haslo", "asd"+gen.nextInt(), gen.nextInt());
            obj.dodajStudenta(name1);
            Grupa n = new Grupa();
            obj.dodajGrupe(n);
            listaOsob lista = new listaOsob(name1, n);
            obj.dodajListeOsob(lista);

Answer 1

dodajListeOsob抛出object references an unsaved transient instance

因为listaOsob包含grupa（@ManyToOne），这在数据库中不存在
grupa在数据库中不存在，因为dodajGrupe不像其他方法那样persist
dodajListeOsob也不会调用persist（在这种情况下不会导致任何问题）

object引用未保存的瞬态实例错误

1 个答案: