protected function validateRequired(array $properties)
    {
        $valid = true;
        foreach ($properties as $property) {
            $value = $this->getPropertyValue($property);
            if (empty($value)) {
                $this->addError($property, sprintf('Property "%s" must not be empty.', $property));
                $valid = false;
            }
        }
        return $valid;
    }