好吧，我正试图把这个查询从mysql翻译成mongodb，因为我需要它来优化我的数据，但是到目前为止我什么也没有实现。。。
mysql查询

SELECT notifications.entity_id 
FROM notifications
INNER JOIN discussions
ON notifications.entity_id = discussions._id 
WHERE notifications.subscribers = 1

不管我怎么努力，我似乎都无法接近另一张table。。。我用php做的很简单，但由于优化程度很低甚至没有优化，这会引起很多麻烦。。。

public function getData($userId) {

    $wheres =   ['subscribers' => $userId];
    $data   =   $this->get($wheres, ['entity_id']); # works for notifications table that I have predefined for this function

    $wheres =   ['_id' => $data['_id']];
    $data  =   $this->get_discussions($wheres, []); #queries discussions table

    return $data;
}