JPA：如何为集合中的项目编写 order by 子句？

Question

我正在使用 JPA 2.0 和 Hibernate 4.1.0.Final。我的 Hibernate 实体中有这个……

@OneToMany(fetch = FetchType.EAGER, cascade = CascadeType.ALL)
@JoinTable(name = "user_address", joinColumns = @JoinColumn(name = "user_id"), inverseJoinColumns =     @JoinColumn(name = "address_id"))
private Set<Address> addresses;

其中Address实体有这个字段：

@Column(name = "email")
private String email;

考虑到用户可能根本没有链接到任何地址对象这一事实，我如何为用户对象的查询编写一个 order by 子句以按此集中的第一个电子邮件地址排序？我试过了

final CriteriaBuilder builder = m_entityManager.getCriteriaBuilder();
final CriteriaQuery<User> criteria = builder.createQuery(User.class);
final Root<User> user = criteria.from(User.class);
...
Expression orderByExpr = orderByRoot.get(orderByCol).get("addresses").get("email");
criteria.orderBy(orderByExpr);

但得到错误...

java.lang.IllegalArgumentException: Unable to resolve attribute [email] against path
    at org.hibernate.ejb.criteria.path.AbstractPathImpl.unknownAttribute(AbstractPathImpl.java:116)
    at org.hibernate.ejb.criteria.path.AbstractPathImpl.locateAttribute(AbstractPathImpl.java:221)
    at org.hibernate.ejb.criteria.path.AbstractPathImpl.get(AbstractPathImpl.java:192)
    at org.mainco.subco.user.repo.UserDaoImpl.addOrderByCol(UserDaoImpl.java:446)
    at org.mainco.subco.user.repo.UserDaoImpl.buildFindUsersQuery(UserDaoImpl.java:342)
    at org.mainco.subco.user.repo.UserDaoImpl.findUsers(UserDaoImpl.java:153)
    at org.mainco.subco.user.repo.UserDao2IT.testFindOrderByEmail(UserDao2IT.java:558)

编辑：这是我按照 Andrei I 推荐的粗略代码。它会导致我在评论中列出的错误。

final CriteriaBuilder builder = m_entityManager.getCriteriaBuilder();
final CriteriaQuery<User> criteria = builder.createQuery(User.class);
final Root<User> user = criteria.from(User.class);
...
final Root<User> user = criteria.from(User.class);
 Join<User, Address> addr = user.join("addresses", JoinType.LEFT);
 orderByExpr = addr.get("email"); 
 criteria.orderBy(builder.asc(orderByExpr));
…            
return criteria.where(builder.and(…))
            .select(user);

Answer 1

尝试以下方法：

final CriteriaQuery<User> query = builder.createQuery(User.class);
final Root<User> user = query.from(User.class);
query.select(user);//this one was missed.

SetJoin<User, Address> addr = user.joinSet("addresses", JoinType.LEFT);

query.orderBy( builder.asc(addr.get("email")) );

PS：由于addresses 字段是一个集合，它没有排序，这意味着不清楚列表中的哪个会被考虑。同样在 SQL 中，您需要一些标准来定义“第一个”元素。