相关疑难解决方法(0)

在我之前的问题的参考,我有一个关于$ lookup的问题,添加一些条件.您可以从以下链接描述中获得有关问题的足够参考.

照片:

{_id: 1, photo_name: '1.jpg', photo_description: 'description 1', album_id: 1, flag:1 },
{_id: 2, photo_name: '2.jpg', photo_description: 'description 2', album_id: 1, flag:1 },
{_id: 3, photo_name: '3.jpg', photo_description: 'description 3', album_id: 1, flag:1 },
{_id: 4, photo_name: '4.jpg', photo_description: 'description 4', album_id: 2, flag:0 },
{_id: 5, photo_name: '5.jpg', photo_description: 'description 5', album_id: 2, flag:0 },
{_id: 6, photo_name: '6.jpg', photo_description: 'description 6', album_id: 2}

专辑:

{_id: 1, album_name: "my album 1", album_description: "album description 1", emoji_id: 1}, …