Symfony 3: UniqueEntity(errorPath) not working - doctrine-orm

Question

I have an entity called ClassSubject and you can see it bellow,
/**
* ClassSubject
*
* #ORM\Table(name="class_subject")
* #ORM\Entity(repositoryClass="PIE10Bundle\Repository\ClassSubjectRepository")
*
* #UniqueEntity( fields={"class", "subjects"},
* errorPath="subjects",
* message="This subject is already added"
* )
*/
class ClassSubject
{
/**
* #var int
*
* #ORM\Column(name="id", type="integer")
* #ORM\Id
* #ORM\GeneratedValue(strategy="AUTO")
*/
private $id;
/**
* #var int
*
* #ORM\ManyToOne(targetEntity="Classes")
* #ORM\JoinColumn(name="class_id", referencedColumnName="id")
*/
private $class;
/**
* #var int
*
* #ORM\ManyToOne(targetEntity="Subject")
* #ORM\JoinColumn(name="subject_id", referencedColumnName="id")
*/
private $subjects;
as you can see, I want to make this Unique Entity so I am following this guide.
Further I have added the followings to the top of the entity file,
use Symfony\Component\Validator\Constraints as Assert;
use Symfony\Bridge\Doctrine\Validator\Constraints\UniqueEntity;
After all when I submit data it still adds duplicating rows to the table. For an example,
if the table exists with a row like, class=clsA and subjects=subA and if I try to insert with same values, it inserts data with no validation/message "This subject is already added"
My controller is like below,
public function addTeacherSubjectAction( $id, Request $request )
{
$em = $this->getDoctrine()->getManager();
$user = $em->getRepository('PIE10Bundle:Users')->find($id);
$subjects = $em->getRepository('PIE10Bundle:Subject')->findAll();
$form = $this->createForm(ClassSubjectType::class,$subjects);
$form->handleRequest($request);
if( $form->isSubmitted() && $form->isValid() )
{
$allSubjects = $form['subject']->getData();
foreach( $allSubjects as $subject )
{
$subject_repo = $em->getRepository('PIE10Bundle:Subject')->find($subject->getId());
$teacherSubject = new TeacherSubjects();
$teacherSubject->setTeachers($user);
$teacherSubject->setSubjects($subject_repo);
$em->persist($teacherSubject);
$em->flush();
}
$this->addFlash('notice',
'Subjects added');
return $this->redirectToRoute('user_teachers');
}
return $this->render( 'PIE10Bundle:Form:layout_commom_form.html.twig',
array( 'title' => 'Add Subjects',
'form' => $form->createView() )
);
}
and the ClassSubjectType is like below,
class ClassSubjectType extends AbstractType
{
public function buildForm(FormBuilderInterface $builder, array $options)
{
$builder->add('subject',
EntityType::class,
array('class' => 'PIE10Bundle:Subject',
'expanded' => true,
'multiple' => true,
'attr' => array( 'class' => 'form-control',
'style' => 'margin:5px 0;')));
$builder->add('Add Classes',
SubmitType::class,
array('attr' => array('class' => 'btn btn-primary',
'style' => 'margin:15px 0;')) );
}
}
So I need to know what is wrong with my code. Thanks in advance.