更新状态，城市下拉，根据国家选择Symfony

我有这个关系。国与国之间的一对多。州与市之间一对多。

在就业表，我必须显示这些下拉，我想当我选择一个国家，只有它的相关国家显示。

这是我的表单代码。

<?php
namespace PNC'UsersBundle'Form;
use Symfony'Component'Form'AbstractType;
use Symfony'Component'Form'FormBuilderInterface;
use Symfony'Component'OptionsResolver'OptionsResolver;
use Symfony'Component'Form'FormBuilder;
use Doctrine'ORM'EntityRepository;
use Symfony'Component'Form'FormEvents;
use Symfony'Component'Form'Event'DataEvent;
class EmploymentsType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder ->add('designation', 'text', array(
                    'required' => true,
                    'label' => 'Designation ',
                    'attr' => array(
                        'required'    => true,
                        'placeholder' => 'Designation Name',
                        'invalid_message' => 'You entered an invalid designation',
                        'class' => 'form-control'
                    )))
                    ->add('organization', 'text', array(
                    'label' => 'Organization',
                    'attr' => array(
                    'required'    => true,
                    'placeholder' => 'Organization Name',
                    'invalid_message' => 'You entered an invalid organization',
                    'class' => 'form-control'
                    )))
                    ->add('country', 'entity', array(
                        'label' => ucfirst('country'),
                        'class' => 'PNCGeneralBundle:Country',
                        'property'=>'name',
                        //'property_path'=>false, //Country is not directly related to City
                        'attr'=> array(
                            'required'    => true,
                            'invalid_message' => 'You selected an invalid Industry',
                            'class' => 'form-control'
                        )
                    ))
                    ->add('state', 'entity', array(
                        'label' => ucfirst('state'),
                        'class' => 'PNC'GeneralBundle'Entity'State',
                        'attr'=> array(
                            'required'    => true,
                            'invalid_message' => 'You selected an invalid Industry',
                            'class' => 'form-control'
                        )
                    ))
                    ->add('city', 'entity', array(
                        'label' => ucfirst('city'),
                        'class' => 'PNC'GeneralBundle'Entity'City',
                        'attr'=> array(
                            'required'    => true,
                            'invalid_message' => 'You selected an invalid Industry',
                            'class' => 'form-control'
                        )
                    ))
                    ->add('save', 'submit', array(
                        'attr' => array(
                            'label' => ucfirst('save'),
                            'class' => 'btn btn-success'
                        )
                    ));
    }
    public function configureOptions(OptionsResolver $resolver)
    {
        $resolver->setDefaults(array(
            'data_class' => 'PNC'UsersBundle'Entity'Employments',
        ));
    }
    public function getName()
    {
        return 'pncusers_bundle_employments_type';
    }
}