JPA多次连接 [英] JPA multiple joins

问题描述

我有这些课程

class Project {
    @ManyToOne Company owner;
    @ManyToMany Set<Person> resources;
}
class Company {
    @ManyToOne Country country;
}
class Person {
}

我如何编写JPQL以获得在特定国家/地区的公司的项目中使用的所有资源?

How can I write a JPQL to get the all the resources working on projects for companies in a specific country?

下面的那个似乎不起作用(使用DataNucleus)

The one below doesn't seem to work (using DataNucleus)

SELECT r FROM Project p JOIN p.resources r JOIN p.owner c WHERE c.country = :country

它尝试将r与c联接，并且当然不具有owner属性，并且在DataNucleus内部引发了NullPointerException.

It tries to join r with c and of course does not have the owner property and a NullPointerException is raised inside DataNucleus.

推荐答案

这应该可以完成您期望的操作:

This should do what you expect:

SELECT resource FROM Project p INNER JOIN p.resources as resource

我忘记了最初问题的一部分:

I forgot a part of the initial question:

SELECT resource FROM Project p INNER JOIN p.resources as resource where p.owner.country = :country

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