public function group($keys, array $initial, $reduce = null, array $options = [])
    {
        $this->query['type'] = Query::TYPE_GROUP;
        $this->query['group'] = [
            'keys' => $keys,
            'initial' => $initial,
            'reduce' => $reduce,
            'options' => $options,
        ];
        return $this;
    }