官网的案例： https://docs.mongodb.com/manual/reference/operator/aggregation/accumulator/#mongodb-group-grp.-accumulator
数据
db.restaurants.insertMany([
{ "_id" : 1, "name" : "Food Fury", "city" : "Bettles", "cuisine" : "American" },
{ "_id" : 2, "name" : "Meal Macro", "city" : "Bettles", "cuisine" : "Chinese" },
{ "_id" : 3, "name" : "Big Crisp", "city" : "Bettles", "cuisine" : "Latin" },
{ "_id" : 4, "name" : "The Wrap", "city" : "Onida", "cuisine" : "American" },
{ "_id" : 5, "name" : "Spice Attack", "city" : "Onida", "cuisine" : "Latin" },
{ "_id" : 6, "name" : "Soup City", "city" : "Onida", "cuisine" : "Chinese" },
{ "_id" : 7, "name" : "Crave", "city" : "Pyote", "cuisine" : "American" },
{ "_id" : 8, "name" : "The Gala", "city" : "Pyote", "cuisine" : "Chinese" }
])
db.restaurants.aggregate([
{
$group :
{
_id : { city: "$city" },
restaurants:
{
$accumulator:
{
init: function(city, userProfileCity) {
return {
max: city === userProfileCity ? 3 : 1,
restaurants: []
}
},
initArgs: ["$city","Onida"],
accumulate: function(state, restaurantName) {
if (state.restaurants.length < state.max) {
state.restaurants.push(restaurantName);
}
return state;
},
accumulateArgs: ["$name"],
merge: function(state1, state2) {
return {
max: state1.max,
restaurants: state1.restaurants.concat(state2.restaurants).slice(0, state1.max)
}
},
finalize: function(state) {
return state.restaurants
},
lang: "js"
}
}
}
}
])
将上面的案例直接转换成真正的JS代码 == 更加的易懂
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8" />
<meta http-equiv="X-UA-Compatible" content="IE=edge" />
<meta name="viewport" content="width=device-width, initial-scale=1.0" />
<title>Document</title>
</head>
<body></body>
<script>
var objs = [
{ _id: 1, name: "Food Fury", city: "Bettles", cuisine: "American" },
{ _id: 2, name: "Meal Macro", city: "Bettles", cuisine: "Chinese" },
{ _id: 3, name: "Big Crisp", city: "Bettles", cuisine: "Latin" },
{ _id: 4, name: "The Wrap", city: "Onida", cuisine: "American" },
{ _id: 5, name: "Spice Attack", city: "Onida", cuisine: "Latin" },
{ _id: 6, name: "Soup City", city: "Onida", cuisine: "Chinese" },
{ _id: 7, name: "Crave", city: "Pyote", cuisine: "American" },
{ _id: 8, name: "The Gala", city: "Pyote", cuisine: "Chinese" },
];
//分组 == 按city的值
var groups = {};
objs.forEach((item, index) => {
if (!groups[item.city]) {
groups[item.city] = [];
}
groups[item.city].push(item);
});
console.log("=============分组开始==============")
console.log(groups);
console.log("=============分组结束==============")
//==========================MongoDB的聚合【直接粘贴复制的】==============================================
function init(city, userProfileCity) {
let state = {
max: city === userProfileCity ? 3 : 1,
restaurants: [],
};
return state;
}
function accumulate(state, restaurantName) {
if (state.restaurants.length < state.max) {
state.restaurants.push(restaurantName);
}
return state;
}
function merge(state1, state2) {
return {
max: state1.max,
restaurants: state1.restaurants
.concat(state2.restaurants)
.slice(0, state1.max),
};
}
function finalize(state) {
return state.restaurants
}
//========================================================================
var initGroups = {};
for( let groupName in groups) {
var groupElems = groups[groupName];
initGroups[groupName] = JSON.parse(JSON.stringify(groupElems));
var initGroupElems = initGroups[groupName];
for(var index = 0 ; index < groupElems.length; index++) {
//等价于 initArgs: ["$city","Onida"]
let state = init(groupName, "Onida");
//等价于accumulateArgs: ["$name"]
initGroupElems[index] = accumulate(state, groupElems[index]["name"])
}
//等价于merge
var metgeResult = initGroupElems.reduce( merge );
//等价于finalize
initGroups[groupName] = finalize(metgeResult);
}
console.log("=============分组聚合后的结果==开始============")
console.log(initGroups);
console.log("=============分组聚合后的结果==结束============")
</script>
</html>
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