node.js - 无法在 Nodejs 的 mongoose 中使用聚合

Question

我在 mongodb 中有两个集合，名为“users”和“posts”

用户看起来像这样:-

[
    {
      "_id": "5e3ffba65bad3a38dc0509cd",
      "name": "Ekansh Jain",
      "email": "ejekanshjain@gmail.com",
      "password": "$2a$10$lF1evuJgfC0N7fIC4I7KrOG.lq56aiHzndIYwc/YRb3auLTgHfmGK",
      "createdAt": "2020-02-09T12:31:34.117Z",
      "updatedAt": "2020-02-09T12:31:34.117Z",
      "__v": 0
    },
    {
      "_id": "5e403dcd1000793694bd2276",
      "name": "Ekansh Jain",
      "email": "ejekanshjain2@gmail.com",
      "password": "$2a$10$qGmng4KrXVXKt0RLnSMgMueo.BenKM43QIpURrzxW4M.HGDHe//TC",
      "createdAt": "2020-02-09T17:13:49.508Z",
      "updatedAt": "2020-02-09T17:13:49.508Z",
      "__v": 0
    }
]

帖子看起来像这样

[
    {
      "_id": "5e403aa22a28e430b0d2641a",
      "title": "My Post 1",
      "body": "this is my first post's body",
      "createdBy": "5e3ffba65bad3a38dc0509cd",
      "createdAt": "2020-02-09T17:00:18.991Z",
      "updatedAt": "2020-02-09T17:00:18.991Z",
      "__v": 0,
      "userInfo": []
    },
    {
      "_id": "5e403e8b1000793694bd2278",
      "title": "LOL",
      "body": "LOL",
      "createdBy": "5e403dcd1000793694bd2276",
      "createdAt": "2020-02-09T17:16:59.513Z",
      "updatedAt": "2020-02-09T17:16:59.513Z",
      "__v": 0,
      "userInfo": []
    },
    {
      "_id": "5e46c8cfe56d4325b4e8d334",
      "title": "test Post",
      "body": "this is my first post's body",
      "createdBy": "5e3ffba65bad3a38dc0509cd",
      "createdAt": "2020-02-14T16:20:31.771Z",
      "updatedAt": "2020-02-14T16:20:31.771Z",
      "__v": 0,
      "userInfo": []
    }
]

我的 Nodejs Express 应用程序中有两个 Mongoose 模型，分别名为 User 和 Post。

我无法聚合(无法对帖子和用户应用内部联接)

这是我当前正在使用的代码:-

const posts = await Post.aggregate([
            {
                "$lookup": {
                    "from": "users",
                    "localField": "createdBy",
                    "foreignField": "_id",
                    "as": "userInfo"
                }
            }
        ]).exec()

在结果中，它返回带有空 userInfo 对象的帖子对象数组。有人可以帮我解决这个问题吗...

Answer 1

由于 createdBy 是一个字符串，而 _id 的类型是 ObjectId()，那么您需要将两个字段设置为相同的类型，请尝试以下操作:

Post.aggregate([
    {
        $lookup:
        {
            from: "users",
            let: { createdBy: "$createdBy" },
            pipeline: [
                {
                    $match:
                    {
                        $expr:
                            { $eq: [{ $toObjectId: "$$createdBy" }, "$_id"] }
                    }
                }
            ],
            as: "userInfo"
        }
    }
])

测试: MongoDB-Playground

注意:或者，如果您不想每次需要将 createdBy 存储为 ObjectId() 时都进行转换。

因此，在您的 Post 模型中，createdBy 类型应为 Schema.Types.ObjectId。

保存Post文档时createdBy:mongoose.Types.ObjectId('someString');