lithium\data\source\MongoDb::group PHP Method

    public function group($group, $context)
    {
        if (!$group) {
            return;
        }
        if (is_string($group) && strpos($group, 'function') === 0) {
            return array('$keyf' => new MongoCode($group));
        }
        $group = (array) $group;
        foreach ($group as $i => $field) {
            if (is_int($i)) {
                $group[$field] = true;
                unset($group[$i]);
            }
        }
        return array('key' => $group);
    }