Using reference to entity in query_builder option

PollRespondType.php

<?php

namespace Acme\PollBundle\Form\Type;

use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\OptionsResolver\OptionsResolverInterface;

use Acme\PollBundle\Entity\FieldRepository;

class PollRespondType extends AbstractType {
    
    public function buildForm(FormBuilderInterface $builder, array $options) {
        $poll = $options['data'];
        $builder->add('fields', 'entity', array(
            'class'         => 'Acme\PollBundle\Entity\FieldRepository',
            'expanded'      => true,
            'multiple'      => false,
            'property'      => 'title',
            'query_builder' => function(FieldRepository $er) use ($poll) {
                return $er->createQueryBuilder('f')
                    ->where('f.poll = :poll')
                    ->orderBy('f.id', 'ASC')
                    ->setParameter('poll', $poll);
            },
        ));
    }
    
    public function setDefaultOptions(OptionsResolverInterface $resolver) {
        $resolver->setDefaults(array(
            'data_class' => $this->class,
        ));
    }
    
    public function getName() {
        return 'acmel_poll_respond_type';
    }
}