public function id(array $values)
    {
        $count = count($values);
        $ids = array_filter($values, 'ctype_digit');

        if ($count != count($ids)) {
            return $this->translation->text('@field has invalid value', array(
                '@field' => $this->translation->text('Condition')));
        }

        $existing = array_filter($values, function ($user_id) {
            if ($user_id == 0) {
                return true; // 0 also valid if we check that user is logged in
            }
            $user = $this->user->get($user_id);
            return isset($user['user_id']);
        });

        if ($count != count($existing)) {
            return $this->translation->text('@name is unavailable', array(
                '@name' => $this->translation->text('User')));
        }

        return true;
    }