通过 ID 在嵌套数组中查找单个记录，并根据 MongoDB 中的另一个嵌套数组进行排序答案

【问题标题】：Find a single record in a nested array by ID and sort based on another nested array in MongoDB通过 ID 在嵌套数组中查找单个记录，并根据 MongoDB 中的另一个嵌套数组进行排序
【发布时间】：2020-05-05 14:57:06
【问题描述】：

我在 MongoDB 中有以下对象，我的后端使用的是 nodeJS。从数据库中检索数据。 "_id": "idq2" 和 "_id": "id1" 都在 API 正文中提供给我们。

questions = [
    {
        "_id": "idq1"
        "questiontext": "some question",
        "author": "auth2",
        "Answers": []
    },
    {
        "_id": "idq2"
        "questiontext": "some question",
        "author": "auth1",
        "Answers": [
            {
                "_id": "id1",
                "author": "auth1",
                "comments" [
                    {
                        "author": "auth1",
                        "comment": "some comments",
                        "rate": 1,
                        "_id": "idc1"
                    }
                    , {
                        "comment": "some comments",
                        "_id": "idc2"
                        "rate": 3
                    }
                    , {
                        "comment": "some comments",
                        "_id": "idc3"
                        "rate": 2
                    }
                    , {
                        "comment": "some comments",
                        "_id": "idc4",
                        "rate": 5
                    }
                ]

            },
            {
                "_id": "id2",
                "author": "auth2",
                "comments" []
            }
        ]
    }]

假设我有“_id”：“idq2”和“_id”：“id1”，我正在尝试获取下面的对象。

预期结果：

Answer = {
    "_id": "id1",
    "author": "auth1",
    "comments" [
        {
            "author": "auth1",
            "comment": "some comments",
            "rate": 1,
            "_id": "idc1"
        },
        {
            "comment": "some comments",
            "_id": "idc3"
            "rate": 2
        }
        , {
            "comment": "some comments",
            "_id": "idc2"
            "rate": 3
        }

    ]
}

为了让这个问题变得简单：如何获取单个“答案”对象，其中“id1”和前 3 个按速率参数按升序排序的 cmets，该对象应该是单个对象。

【问题讨论】：

所以你收集了两个文档"_id": "idq1" 和"_id": "idq2" 得到"_id": "id1", "author": "auth1" 的过滤器是什么？
我从 API 正文中获取 id，所以我们可以假设我们有这两个 id
所以你的意思是说"_id": "idq2" & Answers array id :: "_id": "id1" ？但是你的问题怎么会说"_id": "idq1"？
是的，我们有这两个 id，我编辑问题以避免混淆:)

标签： mongodb mongoose mongodb-query aggregation-framework

【解决方案1】：

您需要为此尝试aggregation-pipeline：

db.collection.aggregate([
    /** Filter docs with required criteria */
    {
      $match: { "_id": "idq2", "Answers._id": "id1" }
    },
    /** get only one needed element from `Answers` array & now `Answers` will be an object */
    {
      $project: {
        _id: 0,
        Answers: { $arrayElemAt: [ { $filter: { input: "$Answers", cond: { $eq: [ "$$this._id", "id1" ] } } }, 0 ] }
      }
    },
    /** Unwind `comments` array to do sorting */
    {
      $unwind: "$Answers.comments"
    },
    {
      $sort: {
        "Answers.comments.rate": 1
      }
    },
    /** group back based on `Answers._id` & push comments back to `comments` array */
    {
      $group: {
        _id: "$Answers._id",
        author: { $first: "$Answers.author" },
        comments: { $push: "$Answers.comments" }
      }
    }
  ])

测试： mongoplayground

注意：如果没有文档与$match 条件匹配，则此查询将返回一个空数组[]，否则将在响应中返回一个包含一个对象的数组。请记住.aggregate() 将始终返回一个数组。

【讨论】：