一个JPQL查询中的多个连接提取

Question

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 */
}

我希望使用JPQL查询获取所有已加入主题的类别并加入帖子.我写的查询如下:

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

但我得到了错误

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

我发现了有关此错误的文章,但这些文章仅描述了在一个实体中要加入的两个集合的情况.我的问题有点不同,我不知道如何解决它.

可以在一个查询中执行吗？

抱歉我的英语不好,但我通常用其他语言说话

Answer 1

您可以使用Child-Parent提取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 ...

Answer 2

这是复杂联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);