JPA：如何爲集合中的項目編寫訂單子句？

Question

我使用JPA 2.0和Hibernate 4.1.0.Final。我有這個在我的Hibernate實體...

@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; 

在其地址實體擁有這一領域：

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

如何by子句編寫順序對用戶對象的查詢第一訂購在這個集合中的電子郵件地址，考慮到用戶可能根本沒有鏈接到任何地址對象？我曾嘗試

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); 

但得到的錯誤...

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) 

編輯：這裏是粗略的代碼，我在我的評論中列出的錯誤建議，安德烈I.它的結果。

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

嘗試以下操作：

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：作爲addresses字段是一組，它不下令，這意味着它並不清楚哪一個列表將被考慮。同樣在SQL中，您需要一些定義「第一個」元素的標準。