MongoDB，计算几个数组中每个项目的出现次数答案

【问题标题】：MongoDB, counting the occurrences of EACH item in several arraysMongoDB，计算几个数组中每个项目的出现次数
【发布时间】：2021-05-21 14:13:35
【问题描述】：

我对 Mongo 还很陌生，如果这有一些我没有掌握的简单答案，我很抱歉。

我希望能够计算每个数字（流派 ID）出现在所有“movie_genres”数组中的次数，并获取每个数字的计数。我的最终目标是统计某个类型编号在用户帐户中出现的次数（包括所有电影）。

我希望找回这个用户有 2 个28，2 个53，1 个18

电影可以一直添加到用户的“movies_watched”数组中。

const userSeed = [
  {
    email: "rambo@hotmail.com",
    password: "12345",
    movies_watched: [
      {
        title: "Rambo",
        movie_id: 7555,
        movie_runtime: 99,
        movie_genres: [28, 53],
      },
      {
        title: "Rambo: Last Blood",
        movie_id: 522938,
        movie_runtime: 99,
        movie_genres: [28, 53, 18],
      },
    ],

【问题讨论】：

标签： javascript arrays mongodb mongodb-query mongoose-schema

【解决方案1】：

您需要使用聚合和设置（仅唯一值）

查询：

db.getCollection('movies')
    .aggregate([
        // optionaly filter records to apply on
        //{
        //    $match: { email: /rambo/ }
        //},
        {
            $project: { // project values you want to see in result
                // record id is present by default
                email: "$email",
                genres: { // brand new field with desired output
                    $reduce: { // reduce like Array.reduce in javascript
                        input: "$movies_watched", // array to reduce
                        initialValue: [],
                        in: {
                            $setUnion: // the function doing the magic
                            // if you use $concatArrays instead - it will contain duplicities
                            [
                                "$$value", // destination in initialValue
                                "$$this.movie_genres" // field to take items from
                            ]
                        }
                    }
                }
            }
        }
    ])

结果：

[
  {
    "_id": {"$oid": "60a7c3d23541f3714eb91332"},
    "email": "rambo@hotmail.com",
    "genres": [18, 28, 53]
  },
  {
    "_id": {"$oid": "60a7c4373541f3714eb9135f"},
    "email": "dexter@hotmail.com",
    "genres": [13, 18, 28, 53]
  }
]

数据集：

[
    {
        "_id" : ObjectId("60a7c3d23541f3714eb91332"),
        "email" : "rambo@hotmail.com",
        "password" : "12345",
        "movies_watched" : [ 
            {
                "title" : "Rambo",
                "movie_id" : 7555,
                "movie_runtime" : 99,
                "movie_genres" : [ 
                    28, 
                    53
                ]
            }, 
            {
                "title" : "Rambo: Last Blood",
                "movie_id" : 522938,
                "movie_runtime" : 99,
                "movie_genres" : [ 
                    28, 
                    53, 
                    18
                ]
            }
        ]
    },
    {
        "_id" : ObjectId("60a7c4373541f3714eb9135f"),
        "email" : "dexter@hotmail.com",
        "password" : "123456",
        "movies_watched" : [ 
            {
                "title" : "Dexter",
                "movie_id" : 444,
                "movie_runtime" : 99,
                "movie_genres" : [ 
                    13, 
                    18
                ]
            }, 
            {
                "title" : "Rambo: Last Blood",
                "movie_id" : 522938,
                "movie_runtime" : 99,
                "movie_genres" : [ 
                    28, 
                    53, 
                    18
                ]
            }
        ]
    }
]

【讨论】：