Java 一个 JPQL 查询中的多个 JOIN FETCH
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Multiple JOIN FETCH in one JPQL query
提问by Zaprogramowany
I have below entities:
我有以下实体：
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:
并且我想使用 JPQL 查询获取所有带有连接主题和连接帖子的类别。我写了如下查询：
SELECT c FROM Category c JOIN FETCH c.topics t JOIN FETCH t.posts p WHERE ...
But I got the error
但我得到了错误
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.
我找到了有关此错误的文章，但这些文章仅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。
It is possible to do in one query?
可以在一个查询中完成吗？
Sorry for my bad english, but I usually speak in other language
抱歉我的英语不好，但我通常会说其他语言
采纳答案by Vlad Mihalcea
You can use a Child-Parent fetch strategyand recombine the entity tree from the result.
您可以使用Child-Parent fetch 策略并从结果中重新组合实体树。
SELECT p
FROM Post p
JOIN FETCH p.topic t
JOIN FETCH t.category c
WHERE ...
回答by wangf
Here is a working example of complex join and multiple consition:
这是复杂连接和多重条件的工作示例：
String query_findByProductDepartmentHospital = "select location from ProductInstallLocation location "
+ " join location.product prod " + " join location.department dep "
+ " join location.department.hospital hos " + " where prod.name = :product "
+ " and dep.name.name = :department " + " and hos.name = :hospital ";
@Query(query_findByProductDepartmentHospital)
ProductInstallLocation findByProductDepartmentHospital(@Param("product") String productName,@Param("department") String departName, @Param("hospital") String hospitalName);
回答by koly
A workaround is to use @Query and @EntityGraph together, like it mentioned here use @Query and @EntityGraph together
一种解决方法是将@Query 和@EntityGraph 一起使用，就像这里提到的那样将@Query 和@EntityGraph 一起使用
