How to count the results from a Custom Entity using JPA CriteriaQuery - jpa

Question

I am using JPA2.1 criteria API where in I have created a result object using criteria builder construct.
Consider the below sample piece of code.
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);
Below is the main line where I would select.
cq.select(cb.construct(Report.class,
root.get("name"), join1.get("address_type"), join2.get("country")));
Now, I would like to do a count on this construct Report.
I tried count like this.
cq.select(cb.count(cb.construct(......)));
// Failed because count accepts Expression and I tried assigning the cb.construct to Expression which is not working.
How to get the count?

Answer 1

I think it would look something like this:
Your code:
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();
}
As also described in this post: JPA + Hibernate count(*) using CriteriaBuilder - with generatedAlias