通过ID查找嵌套数组中的一条记录，并根据MongoDB中的另一个嵌套数组进行排序。

Question

我在MongoDB中有下面的对象，我的后端使用nodeJS.从DB中检索数据。 "_id": "idq2" 和 "_id": "id1" 是在API体中给我们的。

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", 我试图得到下面的对象.

预期的结果是:

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
        }

    ]
}

把它变成一个简单的问题。如何得到一个单一的 "Answers "对象，其中包含 "id1 "和前3条评论，按rate参数升序排列，该对象应该是一个单一的对象。

Answer 1

你需要尝试 聚合管道 对于这个:

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: 此查询将返回一个空数组 [] 如果没有任何文档符合 $match 标准，否则将在响应中返回一个对象数组。记住 .aggregate() 将始终返回一个数组。