Aggregate function calls cannot be nested

Question

I am consulting the docs from http://www.querydsl.com/static/querydsl/3.2.1/reference/pdf/Querydsl_Reference.pdf

There I see the example

QDepartment department = QDepartment.department;
QDepartment d = new QDepartment("d");
query.from(department)
 .where(department.employees.size().eq(
      new JPASubQuery().from(d).unique(d.employees.size().max())
 )).list(department);

I tried to execute the query using my tables with the code below:

QAdminEntity tableAdmin = QAdminEntity.adminEntity;
JPAQuery query = queryFrom( tableAdmin ).where(
    tableAdmin.id_city.eq( idCity ).and(
        tableAdmin.problems.size().eq(
            subQueryFrom( tableAdmin ).unique(
                tableAdmin.problems.size().min()
            )
        )
    )
);

return query.singleResult( tableAdmin );

That code gives me an error, something like "Aggregate function calls cannot be nested".

I am sorry for my lack of expertise on postgres and querydsl, but I suppose this should be very straightforward.

The resulted query:

select
    adminentit0_.id_admin as id1_3_,
    adminentit0_.ds_email as ds2_3_,
    adminentit0_.ds_name as ds3_3_,
    adminentit0_.ds_password as ds4_3_,
    adminentit0_.ds_username as ds5_3_,
    adminentit0_.id_city as id6_3_,
    adminentit0_.id_permission as id7_3_ 
from
    m_admin adminentit0_ 
where
    adminentit0_.id_city=? 
    and (
        select
            count(problems1_.id_admin) 
        from
            m_problem problems1_ 
        where
            adminentit0_.id_admin=problems1_.id_admin
    )=(
        select
            min(count(problems3_.id_admin)) 
        from
            m_admin adminentit2_,
            m_problem problems3_ 
        where
            adminentit2_.id_admin=problems3_.id_admin
        ) limit ?

As you can see I want to list the admin that have less problems than everyone else from the relation admin -> problems.

How can I make this work?

Adittional question:
Is it necessary to get another "QAdminEntity" field for the subQuery or that "d" variable from the "new QDepartment("d")" code is just for the example?

Answer 1

The following query should have similar semantics

QAdminEntity adminEntity = QAdminEntity.adminEntity;
JPAQuery query = query.from(adminEntity)
    .where(adminEntity.id_city.eq(idCity))
    .orderBy(adminEntity.problems.size().asc())
    .limit(1);