FETCH JOIN 最大深度？

发布于 2025-01-07 14:37:29 字数 480 浏览 1 评论 0原文

W 试图获取超过三个级别的连接：

JOIN FETCH entity1.collection1.collection2  // two OneToMany relations

但是得到：

org.hibernate.HibernateException: Errors in named queries: [...]

是因为它太深了，还是因为无法以这种方式获取集合的集合？我的最大获取深度是 3，如果这是相关的。

同时，我可以从另一侧开始执行三重 JOIN FETCH：

JOIN FETCH entity3.entity2.entity1  // two ManyToOne relations

不知何故，我在 JPA 规范或 Hibernate 文档中找不到任何会限制此子句深度的内容。

原文

W was trying to fetch join over three levels:

JOIN FETCH entity1.collection1.collection2  // two OneToMany relations

but got:

org.hibernate.HibernateException: Errors in named queries: [...]

Is it because it was too deep, or because a collection of collections cannot be fetched this way? My max fetch depth is 3, if this is relevant.

I can, at the same time, do a triple JOIN FETCH starting from the other side:

JOIN FETCH entity3.entity2.entity1  // two ManyToOne relations

Somehow I cannot find anything in JPA specification, or in Hibernate docs, that would limit the depth of this clause.

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贵在坚持 2025-01-14 14:37:30

collection1 的类型为Collection。 Collection 没有 collection2 字段。这就是我对此类查询的推理方式。

您必须在集合上创建显式联接：

select e from Entity1 e
left join fetch e.collection1 as c1
left join fetch c1.collection2 as c2

请注意，这将产生笛卡尔积，因此可能返回大量行。另请注意，只有当两个集合中至少有一个是集合时才有可能。如果它们都是包，Hibernate 在执行查询时会抛出异常。

collection1 is of type Collection. And a Collection doesn't have a collection2 field. That's how I reason about those kind of queries.

You must the create an explicit join over the collection:

select e from Entity1 e
left join fetch e.collection1 as c1
left join fetch c1.collection2 as c2

Note that this will produce a cartesian product, and thus petentially returns a huge number of rows. Also note that it will only be possible if one of the two collections at least is a set. If they're both bags, Hibernate will throw an exception when executing the query.

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