How to use manytomany field in symfony2?

I am stuck at a point from last 2 days. I am having two entities Subjects and Teachers. There is a manytomany relation between two. i have created a manytomany field in teacher entity like this :-

/**
 * @ORM\ManyToMany(targetEntity="College\StudentBundle\Entity\Subjects", inversedBy="assets", cascade={"persist"})
 * @ORM\JoinTable(name="employee_subjects")
 */
 
 private $subjects;
 

which resulted in a third table. My subject entity contains only 3 fields, ID, Department_id and name, Now I want to open a regester form for a teacher where i want add the subjects he can teach, for this i created the subjecttype form ist like this :-

namespace College\StudentBundle\Form;
 use Symfony\Component\Form\AbstractType;
 use Symfony\Component\Form\FormBuilder;
 
 class SubjectsType extends AbstractType
 {
   public function buildForm(FormBuilder $builder, array $options)
   {
    $builder
     ->add('name')
     //  ->add('name', 'entity', array('class' => 'CollegeStudentBundle:Subjects', 'property'=>'name', 'multiple'  => false, ))
      ->add('departmentId', 'entity', array('class' => 'CollegeStudentBundle:Department', 'property'=>'name', 'multiple'  => false, ))
    ;
  } 
 
 
   public function getName()
   {
       return 'college_studentbundle_subjectstype';
   }
 }
 

And here is my teacher form where i included the subject:-

namespace College\HumanResourceBundle\Form;
 
 use College\StudentBundle\Form\SubjectsType;
 use College\StudentBundle\Entity;
 use Symfony\Component\Form\AbstractType;
 use Symfony\Component\Form\FormBuilder;
 
 class HumanResourceType extends AbstractType
 {
   public function buildForm(FormBuilder $builder, array $options)
   {
     $builder
        ->add('empid')
       ->add('joining_date', 'date', array('years' => range(date('Y') - 10, date('Y'))))
       ->add('firstname')
       ->add('nationality')
       ->add('usertype', 'entity', array('class' => 'CollegeUserBundle:Usertype', 'property'=>'name', 'multiple'  => false, ))
 
       ->add('subjects', 'collection', array('type' => new SubjectsType()));
     ;
  }
 
   public function getName()
   {
       return 'College_humanresourcebundle_humanresourcetype';
   }
 }
 
 It is not working properly, I want, when i add new teacher it should display subject at the bottom where i can assign subjects to the teacher which gets saved in manytomany table.
 

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