javascript - MongoDB - mapReduce

Question

我有 mongoDB 集合，其中每个文档看起来像:

{
    "_id": 1,
    "name": "Aurelia Menendez",
    "scores": [{
        "score": 60.06045071030959,
        "type": "exam"
    }, {
        "score": 52.79790691903873,
        "type": "quiz"
    }, {
        "score": 71.76133439165544,
        "type": "homework"
    }]
}

我尝试运行:

db.students.mapReduce(
    function() {
        emit(this._id, this.scores.map(a => a.score));
     },
     function(_id, values) {
        //here i try:
        1) return values.reduce((a, b) => a + b);
        2) return values.reduce((a, b) => a + b, 0);
        3) return Array.sum(values);
     },
    { out: "total_scores" }
 )

我得到什么？我得到每个文档看起来像的集合:

“值”是数组:

{
    "_id": 20,
    "value": [42.17439799514388, 71.99314840599558, 81.23972632069464]
}

“值”是值

{
    "_id": 188,
    "value": "060.314725741828,41.12327471818652,74.8699176311771"
}

“值”是数组

{
    "_id": 193,
    "value": [47.67196715489599, 41.55743490493954, 70.4612811769744]
}

为什么我没有得到元素的总和？当我尝试时 this.scores或 this.scores.score而不是 this.scores.map(a => a.score) ，我有所有属性，或空值。
也许有人有任何想法，我做错了什么？

Answer 1

您应该使用聚合而不是 MapReduce。这是来自官方 Mongo 文件的注释

Aggregation pipeline provides better performance and a more coherentinterface than map-reduce, and various map-reduce operations can berewritten using aggregation pipeline operators, such as $group,$merge, $accumulator, etc..

我用来获得聚合阶段的步骤:

使用 MongoDB Compass 并打开聚合选项卡来测试聚合。

添加阶段: $match :过滤学生， $unwind :压平分数数组， $group :通过总和获得所有分数。

转换为代码

结果是

[
  {
    '$match': {
      'name': 'Aurelia Menendez'
    }
  }, {
    '$unwind': {
      'path': '$scores'
    }
  }, {
    '$group': {
      '_id': '$_id', 
      'total': {
        '$sum': '$scores.score'
      }
    }
  }
]