Symfony2 Choice Form Validator for constraining a minimum selection of choices without using an entity

example.php

<?php

$form = $this->createFormBuilder()
    ->add('test_choice', 'choice', array(
        'expanded' => true,
        'multiple' => true,
        'constraints' => array(new MinChoices(array('min' => 1))), // Require a minimum of 1 selection
        'choices'  => array(
            'environmental_science'            => 'environmental_science',
            'energy'                           => 'energy',
            'international_trade'              => 'international_trade',
            'travel_tourism'                   => 'travel_tourism',
            'restaurant_gastronomy'            => 'restaurant_gastronomy',
            'food_drink'                       => 'food_drink',
            'health_wellbeing_medicine'        => 'health_wellbeing_medicine',
            'education'                        => 'education',
            'personal_development'             => 'personal_development',
            'language_culture'                 => 'language_culture',
            'fashion_beauty'                   => 'fashion_beauty',
            'animals_veterinary'               => 'animals_veterinary',
            'other'                            => 'other',
        )
    ))
    ->add('submit', 'submit', array(
        'label' => 'Speichern',
        'attr'  => array('class' => 'btn btn-primary pull-right')
    ))
    ->getForm();

MinChoices.php

<?php

namespace AppBundle\Validator;

use Symfony\Component\Validator\Constraint;

class MinChoices extends Constraint
{
    const TOO_FEW_ERROR = 2;
    const TOO_MANY_ERROR = 3;

    protected static $errorNames = array(
        self::TOO_FEW_ERROR => 'TOO_FEW_ERROR',
        self::TOO_MANY_ERROR => 'TOO_MANY_ERROR',
    );

    public $min;
    public $max;
    public $minMessage = 'You must select at least {{ limit }} choice.|You must select at least {{ limit }} choices.';
    public $maxMessage = 'You must select at most {{ limit }} choice.|You must select at most {{ limit }} choices.';

    /**
     * {@inheritdoc}
     */
    public function getDefaultOption()
    {
        return 'choices';
    }
}

MinChoicesValidator.php

<?php

namespace AppBundle\Validator;

use Symfony\Component\Validator\Constraint;
use Symfony\Component\Validator\ConstraintValidator;
use Symfony\Component\Validator\Exception\UnexpectedTypeException;

class MinChoicesValidator extends ConstraintValidator
{
    /**
     * {@inheritdoc}
     */
    public function validate($value, Constraint $constraint)
    {
        if (!$constraint instanceof MinChoices) {
            throw new UnexpectedTypeException($constraint, __NAMESPACE__.'\MinChoices');
        }

        if (null === $value) {
            return;
        }

        $count = count($value);

        if ($constraint->min !== null && $count < $constraint->min) {
            $this->buildViolation($constraint->minMessage)
                ->setParameter('{{ limit }}', $constraint->min)
                ->setPlural((int) $constraint->min)
                ->setCode(MinChoices::TOO_FEW_ERROR)
                ->addViolation();

            return;
        }

        if ($constraint->max !== null && $count > $constraint->max) {
            $this->buildViolation($constraint->maxMessage)
                ->setParameter('{{ limit }}', $constraint->max)
                ->setPlural((int) $constraint->max)
                ->setCode(MinChoices::TOO_MANY_ERROR)
                ->addViolation();

            return;
        }
    }
}