[英]JPQL - left join with count for many in one-to-many
花了大約2個小時嘗試了解JPQL查詢為什么沒有返回我期望的結果。 請考慮以下代碼:
System.out.println("JPQL ----------------------------");
Query q = em.createQuery(
"select u.userName, count(p.id) from User u " +
"left join u.posts p group by u.userName");
List x = q.getResultList();
for(Object o : x) {
Object[] y = (Object[])o;
System.out.printf("%s %s\n", y[0], y[1]);
}
System.out.println("Spring Data JPA -----------------");
for(User user : userRepository.findAll()) {
List<Post> posts = postRepository.findAllByAuthor(user);
System.out.printf("%s %s\n", user.getUserName(), posts.size());
}
輸出為:
JPQL ----------------------------
user1 0
user2 0
Spring Data JPA -----------------
user1 3
user2 10
我希望JPQL方法可以打印出與存儲庫方法相同的方法。 哪里錯了?
更新
這是SQL跟蹤顯示的內容:
select
user0_.userName as col_0_0_,
count(post2_.id) as col_1_0_
from User user0_
left outer join User_Post posts1_
on user0_.id=posts1_.User_id
left outer join Post post2_
on posts1_.posts_id=post2_.id
group
by user0_.userName
查詢是正確的,但是關系定義存在問題。 這是我所擁有的:
@Entity
public class User {
@Id
@GeneratedValue(strategy = GenerationType.AUTO)
private Long id;
...
@OneToMany
private List<Post> posts;
...
}
@Entity
public class Post {
@Id
@GeneratedValue(strategy = GenerationType.AUTO)
private Long id;
...
@ManyToOne
private User author;
...
}
看來我必須像這樣指定User
和Post
之間的關系:
@OneToMany(mappedBy = "author")
private List<Post> posts;
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