MongoDB - $accumulator官网Demo转成真正的JS代码 - 方便理解

转x33g5p2x  于2022-02-22 转载在 Go  
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官网的案例： https://docs.mongodb.com/manual/reference/operator/aggregation/accumulator/#mongodb-group-grp.-accumulator

数据

  1. db.restaurants.insertMany([
  2.   { "_id" : 1, "name" : "Food Fury", "city" : "Bettles", "cuisine" : "American" },
  3.   { "_id" : 2, "name" : "Meal Macro", "city" : "Bettles", "cuisine" : "Chinese" },
  4.   { "_id" : 3, "name" : "Big Crisp", "city" : "Bettles", "cuisine" : "Latin" },
  5.   { "_id" : 4, "name" : "The Wrap", "city" : "Onida", "cuisine" : "American" },
  6.   { "_id" : 5, "name" : "Spice Attack", "city" : "Onida", "cuisine" : "Latin" },
  7.   { "_id" : 6, "name" : "Soup City", "city" : "Onida", "cuisine" : "Chinese" },
  8.   { "_id" : 7, "name" : "Crave", "city" : "Pyote", "cuisine" : "American" },
  9.   { "_id" : 8, "name" : "The Gala", "city" : "Pyote", "cuisine" : "Chinese" }
  10. ])

  1. db.restaurants.aggregate([
  2. {
  3.   $group :
  4.   {
  5.     _id : { city: "$city" },
  6.     restaurants:
  7.     {
  8.       $accumulator:
  9.       {
  10.         init: function(city, userProfileCity) {       
  11.           return {
  12.             max: city === userProfileCity ? 3 : 1,    
  13.             restaurants: []                           
  14.           }
  15.         },
  16.         initArgs: ["$city","Onida"],       
  17.         accumulate: function(state, restaurantName) { 
  18.           if (state.restaurants.length < state.max) {
  19.             state.restaurants.push(restaurantName);
  20.           }
  21.           return state;
  22.         },
  23.         accumulateArgs: ["$name"],                    
  24.         merge: function(state1, state2) {
  25.           return {
  26.             max: state1.max,
  27.             restaurants: state1.restaurants.concat(state2.restaurants).slice(0, state1.max)
  28.           }
  29.         },
  30.         finalize: function(state) {                   
  31.           return state.restaurants
  32.         },
  33.         lang: "js"
  34.       }
  35.     }
  36.   }
  37. }
  38. ])

将上面的案例直接转换成真正的JS代码 == 更加的易懂
 

  1. <!DOCTYPE html>
  2. <html lang="en">
  3.   <head>
  4.     <meta charset="UTF-8" />
  5.     <meta http-equiv="X-UA-Compatible" content="IE=edge" />
  6.     <meta name="viewport" content="width=device-width, initial-scale=1.0" />
  7.     <title>Document</title>
  8.   </head>
  9.   <body></body>
  10.   <script>
  11.     var objs = [
  12.       { _id: 1, name: "Food Fury", city: "Bettles", cuisine: "American" },
  13.       { _id: 2, name: "Meal Macro", city: "Bettles", cuisine: "Chinese" },
  14.       { _id: 3, name: "Big Crisp", city: "Bettles", cuisine: "Latin" },
  15.       { _id: 4, name: "The Wrap", city: "Onida", cuisine: "American" },
  16.       { _id: 5, name: "Spice Attack", city: "Onida", cuisine: "Latin" },
  17.       { _id: 6, name: "Soup City", city: "Onida", cuisine: "Chinese" },
  18.       { _id: 7, name: "Crave", city: "Pyote", cuisine: "American" },
  19.       { _id: 8, name: "The Gala", city: "Pyote", cuisine: "Chinese" },
  20.     ];
  21.     //分组 == 按city的值
  22.     var groups = {};
  23.     objs.forEach((item, index) => {
  24.       if (!groups[item.city]) {
  25.         groups[item.city] = [];
  26.       }
  27.       groups[item.city].push(item);
  28.     });
  29.     console.log("=============分组开始==============")
  30.     console.log(groups);
  31.     console.log("=============分组结束==============")
  32.     //==========================MongoDB的聚合【直接粘贴复制的】==============================================
  33.     function init(city, userProfileCity) {
  34.       let state = {
  35.         max: city === userProfileCity ? 3 : 1,
  36.         restaurants: [],
  37.       };
  38.       return state;
  39.     }
  40.     function accumulate(state, restaurantName) {
  41.       if (state.restaurants.length < state.max) {
  42.         state.restaurants.push(restaurantName);
  43.       }
  44.       return state;
  45.     }
  46.     function merge(state1, state2) {
  47.       return {
  48.         max: state1.max,
  49.         restaurants: state1.restaurants
  50.           .concat(state2.restaurants)
  51.           .slice(0, state1.max),
  52.       };
  53.     }
  54.     function finalize(state) {                  
  55.       return state.restaurants
  56.     }  
  57.     //========================================================================
  58.     var initGroups = {};
  59.     for( let groupName in  groups) {
  60.       var groupElems = groups[groupName];
  61.       initGroups[groupName] = JSON.parse(JSON.stringify(groupElems));
  62.       var initGroupElems = initGroups[groupName];
  63.       for(var index = 0 ; index < groupElems.length; index++) {
  64.         //等价于 initArgs: ["$city","Onida"]
  65.         let state = init(groupName, "Onida");
  66.         //等价于accumulateArgs: ["$name"]
  67.         initGroupElems[index] = accumulate(state, groupElems[index]["name"])
  68.       }
  69.       //等价于merge
  70.       var metgeResult = initGroupElems.reduce( merge );
  71.       //等价于finalize
  72.       initGroups[groupName] = finalize(metgeResult);
  73.     }
  74.     console.log("=============分组聚合后的结果==开始============")
  75.     console.log(initGroups);
  76.     console.log("=============分组聚合后的结果==结束============")
  77.    
  78.   </script>
  79. </html>

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原文链接 : https://blog.csdn.net/weixin_39651356/article/details/123058535

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