Question

使用与Spring Data JPA集成的QueryDSL时，我遇到了一种奇怪的行为：

我在Project和Person之间有一个ManyToOne关系。如果我按所有者ID（外键）获取属于用户的所有项目，则一切都按预期工作：

QProject project = QProject.project;
QPerson owner = project.owner;
List<Project> projects = from(project).leftJoin(owner).fetch()
    .where(owner.id.eq(id)).list(project);

生成的查询：

select
    project0_.id as id1_1_0_,
    person1_.id as id1_0_1_,
    project0_.creation_date as creation2_1_0_,
    project0_.name as name3_1_0_,
    project0_.owner as owner4_1_0_,
    person1_.name as name2_0_1_
from
    project project0_
left outer join
    person person1_
        on project0_.owner=person1_.id
where
    project0_.owner=?

但是，假设我们希望通过非外键字段（例如所有者的名字）获取属于某个人的所有项目：

QProject project = QProject.project;
QPerson owner = project.owner; 
List<Project> projects = from(project).leftJoin(owner).fetch()
    .where(owner.name.eq(name)).list(project);

在这些情况下，表Person不必要地连接两次（注意person1_和person2 _）：

select
    project0_.id as id1_1_0_,
    person1_.id as id1_0_1_,
    project0_.creation_date as creation2_1_0_,
    project0_.name as name3_1_0_,
    project0_.owner as owner4_1_0_,
    person1_.name as name2_0_1_
from
    project project0_
left outer join
    person person1_
        on project0_.owner=person1_.id cross
join
    person person2_
where
    project0_.owner=person2_.id
    and person2_.name=?

知道为什么会这样，以及如何避免它？

Answer 1

您需要创建一个别名，以确保在where部分

中重用第一个连接

QProject project = QProject.project;
QPerson owner = new QPerson("owner");
List<Project> projects = from(project)
    .leftJoin(project.owner, owner).fetch()
    .where(owner.name.eq(name))
    .list(project);

Spring Data JPA + QueryDSL查询优化

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