Codeigniter MongoDB擴充之使用Aggregate實現Sum方法

以下就是我在使用 Codeigniter 的 MongoDB 擴充時，添加的一個擴充
就是使用 MongoDB 的 Aggregate 實現 Mysql 中的 Sum 方法

/* Controller.php */
$option = array (
    array (
            '$match' => array (
                    'match_1' => 'value_1',
                    'match_2' => 'value_2'
            )
    ),
    array (
            '$group' => array (
                    '_id'   => null,
                    'sum_1' => ['$sum' => '$amount_1'],
                    'sum_2' => ['$sum' => '$amount_2'],
                    'sum_3' => ['$sum' => '$amount_3']
            )
        )
);
$result = $this -> mongo_db -> aggregate('collection',   $option);
/* Mongo_db.php */
public function aggregate ($collection, $option = array())
{
    try {
            return $this -> db -> {$collection} -> aggregate($option);
    }
    catch (Exception $e) {
            show_error("Unable to aggregate Mongo Databases: {$e->getMessage()}", 500);
    }
}