Symfony2 - dynamically added option in choice, form validation showing field value to null

I want to build a form with choice option, which values will add using ajax and it's working like i want but after submitting form symfony2 showing choice option value to null ?

I tried and successfully added dynamically from generation like cookbook shows - http://ift.tt/1bEX27C

but in my current problem, my field in not depend on other option like described in cookbook - sport-position is depend on sport but in my case option is one and without choices initially and then user add options using ajax ..

i tried this and others - Symfony2 - Dynamic form choices - validation remove but these will work when choice is based on other form option, in my case it depend on itself.

my from class code is this - here

'seachText' is text box using for ajax request and then i added selected option to 'college' choice after selection by user but then after submitting form 'POST_SUBMIT' / 'PRE_BIND', i get $event->getForm()->getData()->getCollege() = null :(

college - entity is like symfony2 default declaration with getter and setter

collegeText - public -no db use, just for ajax field

view image - http://ift.tt/1xTgU7S

<?php

namespace MyBundle\Form;

use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\OptionsResolver\OptionsResolverInterface;
use Symfony\Component\Form\FormEvent;
use Symfony\Component\Form\FormEvents;
use Symfony\Component\Form\FormInterface;
use Symfony\Component\DependencyInjection\ContainerInterface;

class SomethingForm extends AbstractType
{   
    private $container;

    public function __construct(ContainerInterface $container){
        $this->container = $container;
    }

    /**
     * {@inheritdoc}
     */
    public function buildForm(FormBuilderInterface $builder, array $options)
    {   

        $builder->add('searchText','text', array(
                                'label' => 'College',
                                'required' => false,
                                )
                         );

        $formModifier = function (FormInterface $form, $college = null) {
            $form->add('college', 'choice', array(
                                'label' => 'College',
                                'choices' => array(),
                                'multiple' => true
                                )
                            );
        };

        $builder->addEventListener(
            FormEvents::PRE_SET_DATA,
            function (FormEvent $event) use ($formModifier) {
                // this would be your entity, i.e. getCollege
                $data = $event->getData();

                $data ? ($data->searchText = $this->container->get('my.service')->getCollegeBasedOnId($data->getCollege())) : '';

                $formModifier($event->getForm(), null);
            }
        );

        $builder->addEventListener(
            FormEvents::PRE_BIND,
            function (FormEvent $event) use ($formModifier) {

                //here i am getting null if i added values using ajax
                var_dump($event->getForm()->getData());
                die();



                $college = $event->getForm()->getData();
                $formModifier($event->getForm()->getParent(), $college);
            }
        );  
    }

    /**
     * {@inheritdoc}
     */
    public function setDefaultOptions(OptionsResolverInterface $resolver)
    {
        $resolver->setDefaults(array(
            'data_class' => 'MyBundle\Entity\Somthing',
        ));
    }

    /**
     * {@inheritdoc}
     */
    public function getName()
    {
        return 'something';
    }
}