如何使用 JPA CriteriaQuery 计算自定义实体的结果

Question

我正在使用 JPA2.1 标准 API，其中我使用标准构建器构造创建了一个结果对象。

考虑下面的示例代码。

CriteriaBuilder cb;//Assume cb is obtained here 
CriteriaQuery<Report> cq = cb.createQuery(Report.class);// custom selective Class
Root<Student> root = cq.from(Student.class);
Join<Student, XTable> join1 = root.join("xtable", JoinType.INNER);
Join<Student, YTable> join1 = root.join("ytable", JoinType.INNER); 

下面是我要选择的主线。

cq.select(cb.construct(Report.class,
      root.get("name"), join1.get("address_type"), join2.get("country")));

现在，我想计算一下这个构造 Report。

我试过这样计数。

cq.select(cb.count(cb.construct(......)));
// Failed because count accepts Expression and I tried assigning the cb.construct to Expression which is not working.

如何获得计数？

Answer 1

我认为它看起来像这样：

您的代码：

CriteriaBuilder cb;//Assume cb is obtained here 
CriteriaQuery<Report> cq = cb.createQuery(Report.class);// custom selective Class
Root<Student> root = cq.from(Student.class);
Join<Student, XTable> join1 = root.join("xtable", JoinType.INNER);
Join<Student, YTable> join1 = root.join("ytable", JoinType.INNER); 
countItemsByCriteria(entityManagerReference, cq, cb);


private <T> Long countItemsByCriteria(EntityManager em, CriteriaQuery<T> cqEntity, CriteriaBuilder cb) {
    CriteriaBuilder builder = cb;
    CriteriaQuery<Long> cqCount = builder.createQuery(Long.class);
    Root<?> entityRoot = cqCount.from(cqEntity.getResultType());
    cqCount.select(builder.count(entityRoot));
    cqCount.where(cqEntity.getRestriction());
    return em.createQuery(cqCount).getSingleResult();
}

如本文所述：JPA + Hibernate count(*) using CriteriaBuilder - with generatedAlias