Question

我有一个查询如下：

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位用户对该文章进行了投票，但我只需要知道数字并只填充其中的3位即可。

Answer 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，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);
});

这将为您提供如下结果：

[
    {
        "_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
    }
]

Playground

Mongodb-填充项目限制并获取这些项目的总数

1 个答案: