Form/Type/LinkedCustomFieldsType.php
public function appendChoice(FormEvent $event)
    {
        $rootForm = $this->getRootForm($event->getForm());
        $group = $rootForm->getData();
        
        if ($group === NULL) {
            return;
        }

        $choices = array();
        foreach($group->getCustomFields() as $customFields) {
            $choices[$customFields->getSlug()] = 
                    $this->translatableStringHelper
                        ->localize($customFields->getName());
        }
        
        $options = array_merge($this->options, array(
                'choice_list' => new SimpleChoiceList($choices),
            ));
        
        $event->getForm()->getParent()->add($this->choiceName, 'choice', 
                $options);
    }
public function appendChoice(FormEvent $event)
    {
        $rootForm = $this->getRootForm($event->getForm());
        $group = $rootForm->getData();
        
        if ($group === NULL) {
            return;
        }

        $choices = array();
        foreach($group->getCustomFields() as $customFields) {
            $choices[$customFields->getSlug()] = 
                    $this->translatableStringHelper
                        ->localize($customFields->getName());
        }
        
        $options = array_merge($this->options, array(
                'choice_list' => new SimpleChoiceList($choices),
            ));
        
        $event->getForm()->getParent()->add($this->choiceName, 'choice', 
                $options);
    }
append Choice on POST_SET_DATA event

Choices are extracted from custom_field_group (the data associated
with the root form)

@param FormEvent $event
@return void