ConditionalValidationGroup.php

<?php

namespace Incenteev\WebBundle\Validator;

use Symfony\Component\Validator\Constraint;

/**
 * @Annotation
 * @Target({"CLASS", "ANNOTATION"})
 */
class ConditionalValidationGroup extends Constraint
{
    public $groupProvider;

    public function getRequiredOptions()
    {
        return array('groupProvider');
    }

    public function getTargets()
    {
        return self::CLASS_CONSTRAINT;
    }
}

ConditionalValidationGroupValidator.php

<?php

namespace Incenteev\WebBundle\Validator;

use Symfony\Component\PropertyAccess\PropertyAccess;
use Symfony\Component\Validator\Constraint;
use Symfony\Component\Validator\ConstraintValidator;

class ConditionalValidationGroupValidator extends ConstraintValidator
{
    private $propertyAccessor;

    public function __construct()
    {
        $this->propertyAccessor = PropertyAccess::createPropertyAccessor();
    }

    /**
     * @param object|array|null $value
     * @param Constraint        $constraint
     */
    public function validate($value, Constraint $constraint)
    {
        if (null === $value) {
            return;
        }

        /** @var ConditionalValidationGroup $constraint */

        $group = $this->propertyAccessor->getValue($value, $constraint->groupProvider);

        if (is_string($group)) {
            $this->context->validate($value, '', array($group));
        }
    }
}

Prize.php

<?php

namespace Incenteev\WebBundle\Entity;

use Incenteev\WebBundle\Validator as IncenteevAssert;
use Symfony\Component\Validator\Constraints as Assert;

/**
 * @IncenteevAssert\ConditionalValidationGroup(groupProvider="type")
 */
class Prize
{
    const TYPE_CUSTOM = 'custom';
    const TYPE_POINTS = 'points';

    /**
     * @var string
     *
     * @Assert\NotBlank(groups={"custom"})
     * @Assert\Length(max=255, groups={"custom"})
     */
    private $name;

    /**
     * @var string
     *
     * @Assert\NotBlank()
     * @Assert\Choice(callback="getTypeValues")
     */
    private $type = self::TYPE_CUSTOM;

    /**
     * @var float
     *
     * @Assert\NotBlank(groups={"points"})
     * @Assert\Range(min=1, groups={"points"})
     */
    private $value;

    public static function getTypeChoices()
    {
        return array(
            self::TYPE_CUSTOM => 'prize.type.custom',
            self::TYPE_POINTS => 'prize.type.points',
        );
    }

    public static function getTypeValues()
    {
        return array_keys(static::getTypeChoices());
    }

    // Getters and setters ...
}