FETCH JOIN最大深度？ [英] FETCH JOIN maximum depth?

问题描述

  JOIN FETCH entity1.collection1.collection2 //两个OneToMany关系
  

但是：

  org.hibernate.HibernateException：命名查询错误：[...] 
  

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

我可以同时从另一侧开始三JOIN FETCH：

  JOIN FETCH entity3 .entity2.entity1 //两个ManyToOne关系
  

不知何故，我在JPA规范或Hibernate中找不到任何东西docs，这会限制这个子句的深度。

解决方案

collection1 是集合的类型。而集合没有 collection2 字段。这就是我对这些问题的理由。

您必须创建一个明确的连接集合：

 从Entity1中选择e e 
 left join fetch e.collection1 as c1 
 left join fetch c1.collection2 as c2 
  

请注意，这会产生笛卡尔积，因此可能会返回大量的行。还要注意，只有两个集合中的一个至少是一个集合才可能。如果他们都是行李，Hibernate在执行查询时会抛出异常。

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.

解决方案

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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