我如何做一个“深"在 JPQL 中获取连接?

问题描述

我认为我永远不会完全理解获取连接.

I don't think I will ever fully understand fetch joins.

我有一个查询，我试图急切地将引用膨胀"到两个级别.

I have a query where I'm attempting to eagerly "inflate" references down two levels.

也就是说，我的 A 有一个可选的 Collection 的 B，每个 B 要么有 0或 1 个 C.已知 B 集合的大小很小(10-20 个顶部).我想预取这个图表.

That is, my A has an optional Collection of Bs, and each B has either 0 or 1 C. The size of the B collection is known to be small (10-20 tops). I'd like to prefetch this graph.

A 的 B 关系被标记为 FetchType.LAZY 并且是可选的.B 与 C 的关系也是可选的，FetchType.LAZY.

A's B relationship is marked as FetchType.LAZY and is optional. B's relationship to C is also optional and FetchType.LAZY.

我希望我能做到:

SELECT a
  FROM A a
  LEFT JOIN FETCH a.bs // look, no alias; JPQL forbids it
  LEFT JOIN a.bs b // "repeated" join necessary since you can't alias fetch joins
  LEFT JOIN FETCH b.c // this doesn't seem to do anything
 WHERE a.id = :id

当我运行它时，我看到 A 的 B 集合确实被获取了(我在 SQL 中看到一个 LEFT JOIN 引用了B 映射到的表).

When I run this, I see that As B collection is indeed fetched (I see a LEFT JOIN in the SQL referencing the table to which B is mapped).

但是，我没有看到任何证据表明 C 的表被获取.

However, I see no such evidence that C's table is fetched.

如何从给定的 预取所有 C 和所有 B 和所有 C 的可达"一个?我看不出有什么办法可以做到这一点.

How can I prefetch all Cs and all Bs and all Cs that are "reachable" from a given A? I can't see any way to do this.

推荐答案

JPA 规范不允许为 fetch join 设置别名，但一些 JPA 提供程序允许.

The JPA spec does not allow aliasing a fetch join, but some JPA providers do.

EclipseLink 从 2.4 开始执行.EclipseLink 还允许使用点符号(即JOIN FETCH a.bs.c")进行嵌套连接获取，并支持允许嵌套连接的查询提示eclipselink.join-fetch"(您可以指定多个提示相同的提示名称).

EclipseLink does as of 2.4. EclipseLink also allow nested join fetch using the dot notation (i.e. "JOIN FETCH a.bs.c"), and supports a query hint "eclipselink.join-fetch" that allows nested joins (you can specify multiple hints of the same hint name).

一般来说，在获取连接上使用别名时需要小心，因为您可能会影响返回的数据.

In general you need to be careful when using an alias on a fetch join, as you can affect the data that is returned.

看，http://java-persistence-performance.blogspot.com/2012/04/objects-vs-data-and-filtering-join.html

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