规范中的 spring-data 子查询
声明：本页面是StackOverFlow热门问题的中英对照翻译，遵循CC BY-SA 4.0协议，如果您需要使用它，必须同样遵循CC BY-SA许可，注明原文地址和作者信息，同时你必须将它归于原作者（不是我）：StackOverFlow
原文地址: http://stackoverflow.com/questions/12061502/
Warning: these are provided under cc-by-sa 4.0 license. You are free to use/share it, But you must attribute it to the original authors (not me):
StackOverFlow
spring-data subquery within a Specification
提问by John Kroubalkian
Spring-data, Oliver Gierke's excellent library, has something called a Specification(org.springframework.data.jpa.domain.Specification). With it you can generate several predicates to narrow your criteria for searching.
Spring-data是 Oliver Gierke 的优秀库，它有一个叫做Specification(org.springframework.data.jpa.domain.Specification) 的东西。有了它，您可以生成多个谓词来缩小搜索条件。
Can someone provide an example of using a Subquery from within a Specification?
有人可以提供一个在规范中使用子查询的示例吗？
I have an object graph and the search criteria can get pretty hairy. I would like to use a Specification to help with the narrowing of the search, but I need to use a Subquery to see if some of the sub-elements (within a collection) in the object graph meet the needs of my search.
我有一个对象图，搜索条件可能会变得非常繁琐。我想使用规范来帮助缩小搜索范围，但我需要使用子查询来查看对象图中的某些子元素（集合内）是否满足我的搜索需求。
Thanks in advance.
提前致谢。
回答by Pieter
String projectName = "project1";
List<Employee> result = employeeRepository.findAll(
new Specification<Employee>() {
@Override
public Predicate toPredicate(Root<Employee> root, CriteriaQuery<?> query, CriteriaBuilder cb) {
Subquery<Employee> sq = query.subquery(Employee.class);
Root<Project> project = sq.from(Project.class);
Join<Project, Employee> sqEmp = project.join("employees");
sq.select(sqEmp).where(cb.equal(project.get("name"),
cb.parameter(String.class, projectName)));
return cb.in(root).value(sq);
}
}
);
is the equivalent of the following jpql query:
相当于以下 jpql 查询：
SELECT e FROM Employee e WHERE e IN (
SELECT emp FROM Project p JOIN p.employees emp WHERE p.name = :projectName
)
