问题
I am using spring data jpa and querydsl and trapped on how to write simple nice query to left join two tables. Suppose I have an Project entity and a Task entity with OneToMany relationship defined in Project, I would like to do something like:
select * from project p left join task t on p.id = t.project_id where p.id = searchTerm
select * from project p left join task t on p.id = t.project_id where t.taskname = searchTerm
In JPQL, it should be:
select distinct p from Project p left join p.tasks t where t.projectID = searthTerm
select distinct p from Project p left join p.tasks t where t.taskName = searthTerm
I have a ProjectRepository interface, which extends JpaRepository and QueryDslPredicateExecutor. That gives me access to method:
Page<T> findAll(com.mysema.query.types.Predicate predicate, Pageable pageable)
I know that left join can be easily achieved by creating a new JPAQuery(entityManager). But I do not have entity manager explicitly injected with spring data jpa. Is there nice and simple way to build a predicate with left join? Wish someone here have experienced this and is able to give me an example. Thank you.
Frey.
回答1:
If you want to express a constraint on tasks then you can do it like this
QProject.project.tasks.any().id.eq(searchTerm)
If you want to express preloading of certain tasks instead via a left join you can't express that via a Predicate. A Predicate in Querydsl is a boolean expression for the where, join-on and having parts of the query.
来源：https://stackoverflow.com/questions/23077155/left-join-with-spring-data-jpa-and-querydsl