在一个JPQL查询中多次联接提取

问题描述

我有以下实体：

  public class Category {
 private Integer id; 
 @OneToMany（mappedBy =parent）
 private List< Topic>主题; 
} 
 
 public class Topic {
 private Integer id; 
 @OneToMany（mappedBy =parent）
私人列表<文章>帖子; 
 @ManyToOne 
 @JoinColumn（name =id）
私人类别父; 
} 
 
 public class Post {
 private Integer id; 
 @ManyToOne 
 @JoinColumn（name =id）
 private主题parent; 
 / *发布字段* / 
} 
  

我想要获取所有类别使用JPQL查询加入主题并加入帖子。我写了如下所示的查询：

  SELECT c FROM Category c JOIN FETCH c.topics t JOIN FETCH t.posts p WHERE。 .. 
  

但是我收到错误

<$
$ / pre

org.hibernate.loader.MultipleBagFetchException：不能同时获取多个行李


我找到了关于这个错误的文章，但是这些文章只描述了在一个实体中有两个要加入的集合的情况。我的问题有点不同，我不知道如何解决它。

可以在一个查询中完成吗？

对不起，我的英语不好，但我通常用其他语言说话

href =http://vladmihalcea.com/hibernate-facts-multi-level-fetching/ =noreferrer> Child-Parent fetch策略并重新组合结果中的实体树。

 选择p 
 FROM发布p 
 JOIN FETCH p.topic t 
 JOIN FETCH t.category c 
 WHERE ... 
  

I have below entities:

public class Category {
    private Integer id;
    @OneToMany(mappedBy = "parent")
    private List<Topic> topics;
}

public class Topic {
    private Integer id;
    @OneToMany(mappedBy = "parent")
    private List<Posts> posts;
    @ManyToOne
    @JoinColumn(name = "id")
    private Category parent;
}

public class Post {
    private Integer id;
    @ManyToOne
    @JoinColumn(name = "id")
    private Topic parent;
    /* Post fields */
}

and I want fetch all categories with joined topics and joined posts using JPQL query. I was wrote query like below:

SELECT c FROM Category c JOIN FETCH c.topics t JOIN FETCH t.posts p WHERE ...

But I got the error

org.hibernate.loader.MultipleBagFetchException: cannot simultaneously fetch multiple bags

I found articles about this error, but these articles only describe situation where in one entity are two collections to join. My problem is a little different and I don't know How to solve it.

It is possible to do in one query?

Sorry for my bad english, but I usually speak in other language

解决方案

You can use a Child-Parent fetch strategy and recombine the entity tree from the result.

SELECT p 
FROM Post p 
JOIN FETCH p.topic t 
JOIN FETCH t.category c 
WHERE ...

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