JPA

Question

为什么有时需要施法？

Join<X, Y> a = (Join) Fetch<X, Y> ...

例如：

Root<Person> personRoot = criteriaQuery.from(Person.class);

@SuppressWarnings("unchecked")
Join<Person, Loan> loanJoin = (Join) personRoot.fetch("loan", JoinType.INNER);
loanJoin.fetch("borrower", JoinType.LEFT);

没有做的原因是什么：

Fetch<Person, Loan> fetchJoin = personRoot.fetch("loan", JoinType.INNER);
fetchJoin.fetch("borrower", JoinType.LEFT);

Answer 1

当我想获得两者的好处时，我不得不将 Fetch 转换为 Join。

Fetch 允许在注解不起作用时预先加载请参阅
Join 允许在连接关系

例如，假设您想要将所有 Employee 连同有关其部门和祖国的信息作为具有两个内部联接的单个 select 查询来获取。这可以通过为 department 和 homeCountry 添加一个 root.fetch(...) 来实现。如果您还希望根据各自所在国家/地区的人口对员工进行排序（请假设您愿意），您将需要 Join

Root<Employee> root = query.from(Employee.class);
root.fetch("department");  // <- for an eager join
Join<Employee,Country> joinCountry = (Join) root.fetch("homeCountry");  // <- for an eager join & orderBy
query.select(root).orderBy(builder.asc(joinCountry.get("population")));
Query<Employee> q = sessionFactory.getCurrentSession().createQuery(query);
List<Employee> employees = q.getResultList();

以上代码向 db

触发一个 select *

select
    employee0_.emp_id as emp_id1_0_0_,
    department1_.department_id as depart1_2_1_,
    country2_.country_id as countr1_3_2_,
    employee0_.salary as salary2_0_0_,
    department1_.name as name3_0_0,
    country2_.continent as continent4_0_0
from
    employee employee0_
inner join
    department department1_
        on employee0_.department_id=department1_.department_id 
inner join
    country country2_
        on employee0_.country_id=country2_.country_id 
order by
    country2_.population asc

Answer 2

确定的答案在这里：How to properly express JPQL "join fetch" with "where" clause as JPA 2 CriteriaQuery?

基本上，JPA 提供 @OneToMany，需要将 所有相关实体 提取到 Collection。如果您允许别名为 Fetch（如 javax.persistence.criteria.Path.get(String)），则可以添加 where 或 on 表达式来限制 Collection 破坏 JPA Java 模型假设。

@*ToOne 联接没有问题，因此一些 JPA 实现允许将 Fetch 强制转换为 Join 假设您了解自己在做什么))

其他阅读：

Using the JPA Criteria API, can you do a fetch join that results in only one join?
Navigation method/s in criteria fetch joins in JPA 2.1

JPA - 为什么投 Fetch 加入

JPA - Why cast Fetch to Join

jpa-criteria