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node.js - 无法在来自nodejs的猫鼬中使用聚合

问题描述

我在 mongodb 中有两个名为“用户”和“帖子”的集合

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 应用程序中有两个猫鼬模型，名为 User 和 Post。

我无法聚合（无法对帖子和用户应用内部联接）

这是我目前正在使用的代码：-

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

结果，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...

标签： node.jsmongodbmongoosemongodb-queryaggregation-framework

解决方案

由于createdBy字符串 &_id是 type ObjectId()，那么您需要将两个字段都设置为相同的类型，试试这个：

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

测试： MongoDB-游乐场

注意：或者，如果您不想在每次需要存储createdBy为ObjectId().

所以在你的Post模型createdBy类型中应该是Schema.Types.ObjectId.

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

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