Order by #oneToMany field using JPA Specification - spring

Question

Consider the entities below -
#Entity
public class Employee {
#Id
#GeneratedValue
private long id;
private String name;
#OneToMany(mappedBy = "employee", fetch = FetchType.LAZY, cascade = CascadeType.ALL)
private List<Phone> phones; //contains both "active" & "inactive" phones
}
#Entity
public class Phone {
#Id
#GeneratedValue
private long id;
private boolean active;
private String number;
#ManyToOne(fetch = FetchType.LAZY)
private Employee employee;
}
I need to pull all the employees and sort them depending on the count of "active" phones they have.
Please note that the employee can have active as well as inactive phones. So the query I am trying to achieve is
ORDER BY (SELECT
COUNT(phone4_.employee_id)
FROM
phone phone4_
WHERE
employee4_.id = phone4_.employee_id
AND phone4_.active = true
) DESC
I am stuck with specification here because of some reason and below is the code I have used -
List<Order> orders = new ArrayList<>();
orders.add(cb.desc(cb.size(employee.get("phones"))));
cq.orderBy(orders);
When I run the code the query that's get generated is
ORDER BY (SELECT
COUNT(phone4_.employee_id)
FROM
phone phone4_
WHERE
employee4_.id = phone4_.employee_id) DESC
I am unable to add an extra AND condition to the logic. Please suggest

Answer 1

As specified in the Persistence API specification:
4.6.16 Subqueries
Subqueries may be used in the WHERE or HAVING clause.
JPA doesn't support subqueries in the order by clause, nor in the select clause.
Hibernate ORM, though, supports them in the SELECT and WHERE clauses.
So you cannot write that query and being JPA compliant.
This HQL should work though and it's covered by Hibernate ORM:
SELECT e1, (SELECT count(p)
FROM Phone p
WHERE p.active = true AND p.employee = e1) as activeCount
FROM Employee e1
ORDER BY activeCount DESC
Surprisingly, writing this query with criteria doesn't work:
CriteriaBuilder builder = ormSession.getCriteriaBuilder();
CriteriaQuery<Object> criteria = builder.createQuery();
Root<Employee> root = criteria.from( Employee.class );
Subquery<Long> activePhonesQuery = criteria.subquery( Long.class );
Root<Phone> phoneRoot = activePhonesQuery.from( Phone.class );
Subquery<Long> phonesCount = activePhonesQuery
.select( builder.count( phoneRoot ) )
.where( builder.and( builder.isTrue( phoneRoot.get( "active" ) ), builder.equal( phoneRoot.get( "employee" ), root ) ) );
criteria.multiselect( root, phonesCount )
.orderBy( builder.desc( phonesCount ) );
The reason is that, Hibernate ORM tries to expand the subquery in the order by clause instead to refer to an alias. And as I mentioned before, this is not supported.
I think the HQL is the easiest option if you don't want to use native queries.