Unexpected Cross Join Generated SQL Using EntityManager with hibernate - sql

Question

I'm Trying to get the number of user in a specific group, I've done the following :
CriteriaBuilder cb = em.getCriteriaBuilder();
CriteriaQuery<Long> q = cb.createQuery(Long.class);
q.select(cb.count(q.from(User.class))).where(cb.equal(q.from(User.class).get("userGroup"), groupId));
But instead of getting the number of user in a group , I get this number multiplied by the number of all users in my table, the generated ORM request looks like this :
Hibernate: select count(*) as col_0_0_ from app_user user0_ cross join app_user user1_ where user1_.user_group=1
EDIT :
This is my user Model :
public class User {
private String userFirstName;
private String userLastName;
/* some stuff */
#ManyToOne
private Group userGroup;
}
And my group model has an int attribute annotated with #Id and named id. How Can I get the number of user by group id in this case ?

Answer 1

This is the expected behaviour. CriteriaQuery is mutable and every time you call from(), it
Create and add a query root corresponding to the given entity, forming a cartesian product with any existing roots. (Javadoc of Java EE)
To achieve predictable result please create Root once and use the reference.
CriteriaQuery<Long> q = cb.createQuery(Long.class);
Root<User> u = q.from(User.class);
q.select(cb.count(u)).where(cb.equal(u.get("userGroup"), groupId));
Within above code u plays same role as in this query
SELECT u.name FROM User u
Explaining it further, calling from() twice would be equivalent of
SELECT u1, u2 FROM User u1, User u2