# Labone\Bundle\OrderBundle\Form\RequestProductType.php
...
class RequestProductType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder
                ->add('product', 'labone_entity_product', array(
                    'class' => "Labone\Bundle\ProductBundle\Entity\Product",
                    'attr' => array('class' => "chosen_find input-xlarge")
                ))

# Labone\Bundle\OrderBundle\Form\Type\LaboneEntityProductType.php
...
class LaboneEntityProductType extends AbstractType
{

    public function getParent()
    {
        return 'entity';
    }

    public function getName()
    {
        return 'labone_entity_product';
    }

    /**
     * {@inheritdoc}
     */
    public function buildView(FormView $view, FormInterface $form, array $options)
    {
        foreach ($view->vars['choices'] as $choice) {
            $choice->attr = array_replace(isset($choice->attr) ? $choice->attr : array(), array(
                'data-price_unitary' => $choice->data->getPrice()
            ));
        }
    }

}

class EntityTreeType extends AbstractType {

    public function buildView( FormView $view , FormInterface $form , array $options ) {

        $choices = [];

        foreach ( $view->vars['choices'] as $choice ) {
            $choices[] = $choice->data;
        }

        $choices = $this->buildTreeChoices( $choices );

        $view->vars['choices'] = $choices;

    }

    protected function buildTreeChoices( $choices , $level = 0 ) {

        $result = array();

        foreach ( $choices as $choice ){

            $result[] = new ChoiceView(
                str_repeat( '-' , $level ) . ' ' . $choice->getName(),
                $choice->getId(),
                $choice,
                []
            );

            if ( !$choice->getChilds()->isEmpty() )
            $result = array_merge(
                $result,
                $this->buildTreeChoices( $choice->getChilds() , $level + 1 )
            );

        }

        return $result;

    }

    public function setDefaultOptions( OptionsResolverInterface $resolver ) {
        $resolver->setDefaults(array(
            'query_builder' => function ( EntityRepository $repo ) {
                return $repo->createQueryBuilder('e')->where('e.parent IS NULL');
            }
        ));
    }

    public function getParent() {
        return 'entity';
    }

    /**
     * Returns the name of this type.
     *
     * @return string The name of this type
     */
    public function getName()
    {
        return 'entity_tree';
    }
}