public function setGroup($key, $checkLastModified = true)
    {
        $this->_groupKey = $key;
        if ($checkLastModified) {
            if (! $this->groupsConfigFile) {
                $this->groupsConfigFile = self::app()->groupsConfigPath;
            }
            if (is_file($this->groupsConfigFile)) {
                $gc = (require $this->groupsConfigFile);
                $keys = explode(',', $key);
                foreach ($keys as $key) {
                    if (!isset($gc[$key])) {
                        // this can happen if value is null
                        // which could be solved with array_filter
                        continue;
                    }

                    $this->_lastModified = self::getLastModified($gc[$key], $this->_lastModified);
                }
            }
        }
    }