从惰性获取列表中删除-Hibernate

Question

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

    @ManyToMany(fetch = FetchType.LAZY, mappedBy = "attendingUsers")
    private List<Event> attendingEvents = new ArrayList<Event>();
}

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

    @ManyToMany(fetch = FetchType.LAZY)
    @JoinTable(name = "Event_User",
        joinColumns = @JoinColumn(name = "event_id", nullable = false, updatable = false), 
        inverseJoinColumns = @JoinColumn(name = "user_id", nullable = false, updatable = false))
    List<User> attendingUsers = new ArrayList<User>();
}

当然，这些列表有setter和getter。现在我想实现以用户身份不参加这些事件的可能性。因此，我尝试了以下方法：

public void unattend(Event e, User u){
    List<User> attendingUsers = getAttendingUsers(e.getId());
    List<Event> attendingEventsForUser = userService.getEvents(u.getId());
    if(attendingUsers.contains(u)){
        e.getAttendingUsers().clear();
        u.getAttendingEvents().clear();
        attendingUsers.remove(u);
        attendingEventsForUser.remove(e);
        e.getAttendingUsers().addAll(attendingUsers);
        u.getAttendingEvents().addAll(attendingEventsForUser);
        update(e);
        userDAO.update(u);
    }
}

我得到的例外情况如下：

尽管我使用了一些方法来确保执行惰性提取：

public List<User> getAttendingUsers(Long id){
    Query query = sessionFactory.getCurrentSession().createQuery(
            "SELECT u FROM User AS u LEFT JOIN u.attendingEvents AS ae WHERE ae.id = :id");
    query.setParameter("id", id);

    List<User> l = (List<User>)query.list();

    return l;
}

    @Override
public List<Event> getEvents(Long id) {
    Query query = sessionFactory.getCurrentSession().createQuery(
            "SELECT e FROM Event AS e LEFT JOIN e.attendingUsers AS au WHERE au.id = :id");
    query.setParameter("id", id);

    List<Event> l = (List<Event>)query.list();

    return l;
}

public void unattend(Event e, User u){
    Query query = sessionFactory.getCurrentSession().createQuery(
            "SELECT u FROM User AS u LEFT JOIN FETCH u.attendingEvents AS ae WHERE ae.id = :id");
    query.setParameter("id", e.getId());
    List<User> attendingUsers = (List<User>)query.list();

    Query query2 = sessionFactory.getCurrentSession().createQuery(
            "SELECT e FROM Event AS e LEFT JOIN FETCH e.attendingUsers AS au WHERE au.id = :id");
    query2.setParameter("id", u.getId());

    List<Event> attendingEventsForUser = (List<Event>)query2.list();

    if(attendingUsers.contains(u)){
        attendingUsers.remove(u);
        attendingEventsForUser.remove(e);
        e.setAttendingUsers(attendingUsers);
        u.setAttendingEvents(attendingEventsForUser);
        update(e);
        userDAO.update(u);
    }
}

Answer 1

您正在尝试访问事务之外的惰性集合。您可以使用@transaction注释相关的方法，也可以使用fetch联接：

SELECT e FROM Event AS e LEFT JOIN FETCH e.attendingUsers ...

在事务中工作通常更好，因为只有在需要时才提取相关实体。

Edit:异常可能源于'unattend'方法中的if子句，在该子句中可以访问列表:e.getAttendingUsers()。我将把它从控制器移到事务性服务中，比如EventService。