validating dynamic forms in symfony2 returns "this value is not valid" - forms

Question

I have 3 entities: Country, State and City with the following relationships, and i have another entity named Location which have all of theme for setting user location,
i have a form too, for getting user's location data. which should be generated dynamically, because the entities are related to each other, and for example if the state changed, the cities must be changed too.
i used the code below in my listener for doing this:
/**
* #param FormFactoryInterface $factory
* #param EntityManager $om
*/
public function __construct($factory, $om)
{
$this->factory = $factory;
$this->om = $om;
}
public static function getSubscribedEvents()
{
return array(
FormEvents::PRE_BIND => 'preBind',
FormEvents::PRE_SET_DATA => 'preSetData',
);
}
/**
* #param event FormEvent
*/
public function preSetData(FormEvent $event)
{
$company = $event->getData();
if (null === $company)
{
return;
}
// company has country => edit action
if($company->getCountry()){
//retrieve location
$country = $company->getCountry();
$state = $company->getState();
$city = $company->getCity();
if(is_object($state) && is_object($city) && is_object($country))
{
$states = $country->getStates();
$cities = $state->getCities();
}
else
{
$cities = array();
$states = array();
}
}else{
// new location action
$states = array();
$cities = array();
}
$form = $event->getForm();
$this->customizeForm($form, $cities,$states);
}
public function preBind(FormEvent $event)
{
$dataForm = $event->getData();
$countryId = $dataForm['country'];
$stateId = $dataForm['state'];
$cityId = $dataForm['city'];
$country = $this->om
->getRepository('GMBundle:Country')
->find($countryId);
$state = $this->om
->getRepository('GMBundle:State')
->find($stateId);
$city = $this->om
->getRepository('GMBundle:City')
->find($cityId);
if(is_object($state) && is_object($city) && is_object($country))
{
$states = $country->getStates();
$cities = $state->getCities();
}
else
{
$cities = array();
$states = array();
}
$form = $event->getForm();
$this->customizeForm($form, $cities, $states);
}
protected function customizeForm($form, $cities, $states)
{
$form
->add($this->factory->createNamed('state', 'entity', null, array(
'choices' => $states,
'label' => 'state',
'required' => true,
'empty_value' => 'choose state',
'class' => 'GMBundle:State',
)))
->add($this->factory->createNamed('city', 'entity', null, array(
'choices' => $cities,
'label' => 'city',
'required' => true,
'empty_value' => 'choose city',
'class' => 'GMBundle:City',
)))
;
}
when i want to edit my form, i got the error This value is not valid.
it seems that symfony doesn't changed the data, for example if user had selected X as Stated and now wants to select Y for his state, symfony said the chosen city is not valid and must be selected from X's cities, which was his previous choice.
any idea how i can resolve it? or where i'm doing something wrong!?
-thanks