子文档中字段的返回计数和平均值

Question

我有已导入收藏的. json-file。

{
    "_id" : ObjectId("5739ee85daa49f685e316fc6"),
    "id" : 38,
    "title" : "It Takes Two (1995)",
    "genre" : "Comedy",
    "ratings" : [
        {
            "userId" : 26,
            "rating" : 2
        },
        {
            "userId" : 531,
            "rating" : 2
        },
        {
            "userId" : 1054,
            "rating" : 2
        },
        {
            "userId" : 1068,
            "rating" : 2
        },
        {
            "userId" : 1221,
            "rating" : 5
        },
        {
            "userId" : 1434,
            "rating" : 4
        },
        {
            "userId" : 1448,
            "rating" : 1
        },
        {
            "userId" : 1645,
            "rating" : 5
        },
        {
            "userId" : 1647,
            "rating" : 1
        },
        {
            "userId" : 1958,
            "rating" : 3
        },
        {
            "userId" : 2010,
            "rating" : 1
        },
        {
            "userId" : 2042,
            "rating" : 1
        },
        {
            "userId" : 2063,
            "rating" : 1
        },
        {
            "userId" : 2106,
            "rating" : 1
        },
        {
            "userId" : 2116,
            "rating" : 3
        },
        {
            "userId" : 2541,
            "rating" : 5
        },
        {
            "userId" : 2777,
            "rating" : 3
        },
        {
             "userId" : 3013,
             "rating" : 2
        },
        {
             "userId" : 3029,
             "rating" : 2
        },
        {
             "userId" : 3111,
             "rating" : 4
        },
        {
             "userId" : 4387,
             "rating" : 1
        },
        {
             "userId" : 4572,
             "rating" : 5
        },
        {
             "userId" : 5361,
             "rating" : 5
        }
      ]
}

我想做一些地图缩减，以显示所有用户的评论总数及其平均值。

我试过：

 var map = function(){emit(this.ratings.userId, 1);}

 var reduce = function(key, values){var res = 0;
 values.forEach(function(v){ res += 1});
 return {count: res};
 }

 db.movie.mapReduce(map, reduce, { out: "users" });

 db.users.find()
 { "_id" : null, "value" : { "count" : 39 } }

我不知道为什么它会显示\u id“：null。我想这个.ratings.userId是错误的。但是这个.ratings[userId]也不起作用。

我预计会出现以下情况：

userId:10, count:2000
userId:20, count:500

你能帮忙吗？

Answer 1

我找到了解决方案：

    var mapFunction = function() {
                       for (var idx = 0; idx < this.ratings.length; idx++) {
                           var key = this.ratings[idx].userId;
                           var value = {
                                         count: 1,
                                         rating: this.ratings[idx].rating
                                       };
                           emit(key, value);
                       }
                    };


   var reduceFunction = function(keyUSERID, countObjVals) {
                     reducedVal = { count: 0, rating: 0 };

                     for (var idx = 0; idx < countObjVals.length; idx++) {
                         reducedVal.count += countObjVals[idx].count;
                         reducedVal.rating += countObjVals[idx].rating;
                     }

                     return reducedVal;
                  };

   var finalizeFunction = function (key, reducedVal) {

                       reducedVal.avg = reducedVal.rating/reducedVal.count;

                       return reducedVal;

                    };

   db.movies.mapReduce( mapFunction,
                     reduceFunction,
                     {
                       out: "users",
                       finalize: finalizeFunction

                     }

                   )

数据库。用户。find（）为我提供：

    { "_id" : 1, "value" : { "count" : 56, "rating" : 237, "avg" : 4.232142857142857 } }
    { "_id" : 2, "value" : { "count" : 129, "rating" : 479, "avg" : 3.7131782945736433 } }
    { "_id" : 3, "value" : { "count" : 51, "rating" : 199, "avg" : 3.9019607843137254 } }
    { "_id" : 4, "value" : { "count" : 21, "rating" : 88, "avg" : 4.190476190476191 } }
    { "_id" : 5, "value" : { "count" : 198, "rating" : 623, "avg" : 3.1464646464646466 } }

Answer 2

您使用了错误的工具。您需要使用允许访问聚合管道的聚合（）方法。在您的管道中，您需要使用$unWind运算符对“评级”数组进行非规范化。从那里您可以简单地按“userId”对文档进行分组，并使用$sum和$avg累加器运算符，它们分别返回字段的总和和和平均值。

db.movie.aggregate([
    { "$unwind": "$ratings" }, 
    { "$group": { 
        "_id": "$ratings.userId", 
        "count": { "$sum": 1 }, 
        "average": { "$avg": "$ratings.rating" } 
    }}
])