Create sql query in sonata admin - sql

Question

I use sonata admin in my project. i created two admin classes exam and student. i want to create an action in exam class that shows all students that should pass the exam after creating a new exam, this is the sql query that i want:
"Select * from student,exam where student.codeAdministration=exam.codeAdministration"
examAdmin.php:
<?php
namespace Exam\ExamBundle\Admin;
use Sonata\AdminBundle\Admin\Admin;
use Sonata\AdminBundle\Datagrid\ListMapper;
use Sonata\AdminBundle\Datagrid\DatagridMapper;
use Sonata\AdminBundle\Validator\ErrorElement;
use Sonata\AdminBundle\Form\FormMapper;
use Sonata\AdminBundle\Show\ShowMapper;
class ExamAdmin extends Admin
{
////
protected function configureFormFields(FormMapper $formMapper)
{
//////
}
protected function configureDatagridFilters(DatagridMapper $datagridMapper)
{
////
}
protected function configureListFields(ListMapper $listMapper)
{
/////
}
protected function configureShowField(ShowMapper $showMapper)
{
//////
}
}
i tried this query without putting the condition of WHERE but it does not work:
protected function configureListFields(ListMapper $listMapper)
{
$query = $this->modelManager->getEntityManager()->createQuery('SELECT s FROM Exam\ExamBundle\Entity\student s');
$listMapper
->add('student', 'sonata_type_model', array('required' => true, 'query' => $query))
}
How can i create this action with this query??
exam.php:
<?php
namespace Exam\ExamBundle\Entity;
use Doctrine\ORM\Mapping as ORM;
class Exam
{
/**
* #var integer
*/
private $idExam
////////
/**
* #var \Examens\ExamensBundle\Entity\Administration
*/
private $codeAdministration;
//////
public function getIdExam()
{
return $this->idExam;
}
///////
public function setCodeAdministration(\Exam\ExamBundle\Entity\Administration $codeAdministration = null)
{
$this->codeAdministration = $codeAdministration;
return $this;
}
/**
* Get codeAdministration
*
* #return \Exam\ExamBundle\Entity\Administration
*/
public function getCodeAdministration()
{
return $this->codeAdministration;
}
///////
}