Question

我想使用 $lookup 获取其他集合的字段。例如，这是我的 users 集合：

{ "_id" : ObjectId("601c4f89049f9d2cc47eece2"), "username" : "Ahmad", "email" : "aaa@aaa.com", "password" : "$2a$08$fAT1G.db9LjXRV7bOPKNzuosWZ3QGJs2rdHIGmPbWtusDA0Qko47e", "tokens" : [ { "_id" : ObjectId("601d4de52472f01ef4552e77"), "token" : "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJpZCI6IjYwMWM0Zjg5MDQ5ZjlkMmNjNDdlZWNlMiIsImlhdCI6MTYxMjUzMzIyMSwiZXhwIjoxNjEzMTM4MDIxfQ.lEt5mIQkjVCRFvDJsHoK1rwdN-WELSVWMFW3p8itZBU" }, { "_id" : ObjectId("601c4f89049f9d2cc47eece3"), "token" : "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJpZCI6IjYwMWM0Zjg5MDQ5ZjlkMmNjNDdlZWNlMiIsImlhdCI6MTYxMjQ2ODEwNSwiZXhwIjoxNjEzMDcyOTA1fQ.WwKCenn7Qh3C1DhJT7H1Kcqu4ZRl0Z7SYJxBV6v87S8" } ], "createdAt" : ISODate("2021-02-04T19:48:25.024Z"), 
"updatedAt" : ISODate("2021-02-05T13:53:41.730Z"), "__v" : 0 }
{ "_id" : ObjectId("601c954a59477b1d38e8a69e"), "username" : "fred", "email" : "isded@gmail.com", "password" : "$2a$08$nZ2kusIoSdwgjBpcZX4WTe2sO3tnieUcVIDGXiUCofLSvkdSdTRFa", "tokens" : [ { "_id" : ObjectId("601c954a59477b1d38e8a69f"), "token" : "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJpZCI6IjYwMWM5NTRhNTk0NzdiMWQzOGU4YTY5ZSIsImlhdCI6MTYxMjQ4NTk2MiwiZXhwIjoxNjEzMDkwNzYyfQ.bHkhfBlgswerB25za2y3YODk-cbRgGhlcrtr-zaIG9M" } ], "createdAt" : ISODate("2021-02-05T00:46:02.518Z"), "updatedAt" : ISODate("2021-02-05T00:46:02.518Z"), "__v" : 0 }

这里是 posts 集合：

{ "_id" : ObjectId("601c5190939a1e294c688267"), "title" : "abdulila", "content" : "wqd", "category" : "Health", "owner" : ObjectId("601c4f89049f9d2cc47eece2"), "comments" : [ { 
"_id" : ObjectId("601c532a5b746f1ad081c57e"), "content" : "Marchgisio", "owner" : ObjectId("601c4f89049f9d2cc47eece2"), "createdAt" : ISODate("2021-02-04T20:03:54.055Z"), "updatedAt" : ISODate("2021-02-04T20:03:54.055Z") }, { "_id" : ObjectId("601c53445b746f1ad081c57f"), "content" : "Balotelli", "owner" : ObjectId("601c4f89049f9d2cc47eece2"), "createdAt" : ISODate("2021-02-04T20:04:20.967Z"), "updatedAt" : ISODate("2021-02-04T20:04:20.967Z") } ], "createdAt" : ISODate("2021-02-04T19:57:04.902Z"), "updatedAt" : ISODate("2021-02-04T20:04:20.968Z"), "__v" : 2 }
{ "_id" : ObjectId("601c958259477b1d38e8a6a0"), "title" : "ohh mighty spurs", "content" : "harry kane", "category" : "Sports", "owner" : ObjectId("601c954a59477b1d38e8a69e"), "comments" : [ { "_id" : ObjectId("601c958f59477b1d38e8a6a1"), "content" : "eorje", "owner" : ObjectId("601c954a59477b1d38e8a69e"), "createdAt" : ISODate("2021-02-05T00:47:11.461Z"), "updatedAt" : ISODate("2021-02-05T00:47:11.461Z") }, { "_id" : ObjectId("601c95cac3b0131328e29344"), "content" : "qwe", "owner" : ObjectId("601c954a59477b1d38e8a69e"), "createdAt" : ISODate("2021-02-05T00:48:10.207Z"), "updatedAt" : ISODate("2021-02-05T00:48:10.207Z") }, { "_id" : ObjectId("601c95e3af31ab1608abd23f"), "content" : "wef", "owner" : ObjectId("601c954a59477b1d38e8a69e"), "createdAt" : ISODate("2021-02-05T00:48:35.168Z"), "updatedAt" : ISODate("2021-02-05T00:48:35.168Z") } ], "createdAt" : ISODate("2021-02-05T00:46:58.962Z"), "updatedAt" : ISODate("2021-02-05T00:48:35.168Z"), "__v" : 3 }
{ "_id" : ObjectId("601d210fbf41fd262c9172c4"), "title" : "bobo", "content" : "hiv is a lie and in god name we will kill all the jews in balastin XD UwU\n", "category" : "Movies", "owner" : ObjectId("601c954a59477b1d38e8a69e"), "comments" : [ ], "createdAt" : ISODate("2021-02-05T10:42:23.773Z"), "updatedAt" : ISODate("2021-02-05T10:42:23.773Z"), "__v" 
: 0 }
{ "_id" : ObjectId("601d4a2f1f169d3d9cbfcbfe"), "title" : "Abdallah", "content" : "dfsv", "category" : "Health", "owner" : ObjectId("601c954a59477b1d38e8a69e"), "comments" : [ { "_id" : ObjectId("601d50d80678f626105f943e"), "content" : "dvd", "owner" : ObjectId("601c4f89049f9d2cc47eece2"), "createdAt" : ISODate("2021-02-05T14:06:16.941Z"), "updatedAt" 
: ISODate("2021-02-05T14:06:16.941Z") }, { "_id" : ObjectId("601d531fcc80c13fb832af40"), "content" : "svsf", "owner" : ObjectId("601c4f89049f9d2cc47eece2"), "createdAt" : ISODate("2021-02-05T14:15:59.873Z"), "updatedAt" : ISODate("2021-02-05T14:15:59.873Z") } ], "createdAt" : ISODate("2021-02-05T13:37:51.787Z"), "updatedAt" : ISODate("2021-02-05T14:15:59.874Z"), "__v" : 2 }

我的目的是获取 1 个特定帖子的所有评论。然后，对于每条评论 - 我想要它的 content 和所有者 username。

这是我的尝试：

Post.aggregate([
    {
        $match: { _id: mongoose.Types.ObjectId(req.query.postID) },
    },
    {
        $limit: 1,
    },
    {
        $project: {
            _id: 0,
            'comments.owner': 1,
            'comments.content': 1,
        },
    },
]);

所以这给了我内容和所有者 ID。但我想要出现在用户集合中的所有者用户名。我怎么能做到这一点？

Answer 1

我想出了这个管道。我相信它的云会得到改进。

步骤是（在管道中注释）

拆分评论数组，以便我们可以专注于每个评论
查找用户信息
查找带来一个包含一个元素的数组，所以我展开它。
用所有者用户名替换所有者对象 ID。
群组评论
投影所需的字段

我用你的一篇帖子直接在 mongodb 上对此进行了测试。

[
    {
        $match: { _id: ObjectId("601d4a2f1f169d3d9cbfcbfe") },
    },
    {
        $unwind: '$comments' //split comments array so we could concentrate on each one
    },
    {
      $lookup: {
        from: 'user',
        localField: 'comments.owner', // lookup for the user info
        foreignField: '_id',
        as: 'owner'
      }  
    },
    {
        $unwind: '$owner' // the lookup brings an array of one element, so I unwind it.
    },    
    {
      $addFields: {
       'comments.owner': "$owner.username"  //replace the owner object id with the owner username
      }
    },
    {
      $group: {
         _id: '_id', // group comments
         'comments': {$push: '$comments'}
      }
    },
    {
      $project:{
         _id: 0,  // project the needed fields
         'comments.content':1,
         'comments.owner': 1,
      }    
    }
])

具有查找功能的 MongoDB 项目

1 个答案: