Mongodb - 填充项目限制并获取这些项目的总数

Posted 2023-03-10

【中文标题】Mongodb - 填充项目限制并获取这些项目的总数

【英文标题】：Mongodb - populate with limit on items and get total count of those items

【发布时间】：2020-03-23 05:34:20

【问题描述】：

我的查询如下所示：

const articles = await Article.find(query)
    .populate(
      path: 'votedUsers', // array of users id
      select: 'title name username',
      options: 
        limit: 3, 
        sort:  createdAt: -1 ,
      ,
    )
    .exec()

结果：

[
 
   title: 'Article title',
   votedUsers: [,,], // array of populated users with limit of 3
   totalCountVoted: 200 // need to add this field 
 
]

我想查找文章并填充 votedUsers 属性，但限制为 3 个用户，但同时 我需要知道votedUsers 属性中有多少个 id。

例如，可能有 200 个用户对那篇文章进行了投票，但我只需要知道这个数字并只填充其中的 3 个。

【参考方案1】：

您可以使用match、lookup、project 阶段以及slice 和size 运算符尝试以下聚合：

（请注意，lookup from 中的“users”值必须是物理集合名称。）

app.get("/article", async (req, res) => 
  const data = await Article.aggregate([
    
      $match: 
        category: "Category1"
      
    ,
    
      $lookup: 
        from: "users",
        localField: "votedUsers",
        foreignField: "_id",
        as: "users"
      
    ,
    
      $project: 
        title: 1,
        votedUsers:  $slice: ["$users", 3] ,
        totalCountVoted:  $size: "$users" 
      
    
  ]);

  res.send(data);
);

这会给你这样的结果：

[
    
        "_id": "5dded78f8f30c402b0fac309",
        "title": "Article1",
        "votedUsers": [
            
                "_id": "5dded60a84523642bc27f511",
                "__v": 0,
                "name": "User1"
            ,
            
                "_id": "5dded61384523642bc27f512",
                "__v": 0,
                "name": "User2"
            ,
            
                "_id": "5dded61b84523642bc27f513",
                "__v": 0,
                "name": "User3"
            
        ],
        "totalCountVoted": 8
    ,
    
        "_id": "5dded7c18f30c402b0fac30a",
        "title": "Article2",
        "votedUsers": [
            
                "_id": "5dded61b84523642bc27f513",
                "__v": 0,
                "name": "User3"
            ,
            
                "_id": "5dded63c84523642bc27f514",
                "__v": 0,
                "name": "User4"
            ,
            
                "_id": "5dded64484523642bc27f515",
                "__v": 0,
                "name": "User5"
            
        ],
        "totalCountVoted": 8
    
]

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