猫鼬聚合:限制$ group中的记录数 [英] Mongoose Aggregate : limit the number of records in $group

问题描述

我正尝试使用Mongoose Aggregate方法转换此句子:

I am trying to convert this sentence using the Mongoose Aggregate method :

对于具有给定oid的每个玩家，选择玩得最多的游戏". 这是我的游戏架构:

"For each player with given oid, select the game that has been played the most". Here is my Game schema:

gameSchema = new mongoose.Schema({
     game_name:{type:String},
     game_id:{type:String},
     oid:{type: String},
     number_plays:{type:Number,default:0},
    })
Game = mongoose.model('Game', gameSchema);

这是我正在使用的代码:

Here is the code I am using :

var allids = ['xxxxx','yyyy'];
Game.aggregate([
    {$match: {'oid': {$in:allids}}},
    {$sort: {'number_plays': -1}},
    {$group: {
        _id: '$oid', 
        plays:{$push:"$number_plays"}, 
        instructions:{$push:"$game_instructions"}
    }}
], function(err,list){
    console.log(list);
    res.end();
});

上面的代码返回以下内容:

The code above returns the following:

[ { _id: 'yyyy', plays: [ 10,4,5 ] },
  { _id: 'xxxxx',
    plays: [ 28, 14, 10, 4, 0, 0, 0, 0, 0, 0, 0, 0, 0 ] } ]

问题在于它会返回所有游戏，而不是大部分已玩过的游戏.所以我的问题是:是否可以限制$ group中填充的字段?

The problem is that it returns all games, not the one that has been mostly played. So my question is : is it possible to limit the fields populated in the $group ?

推荐答案

您可以使用 $first 以便从已排序管道的每个组中的第一个文档中获取值:

You can use $first to take values from the first doc in each group of a sorted pipeline:

var allids = ['xxxxx','yyyy'];
Game.aggregate([
    {$match: {'oid': {$in:allids}}},
    {$sort: {'number_plays': -1}},
    {$group: {
        _id: '$oid', 
        game_name: {$first: "$game_name"}, 
        game_id: {$first: "$game_id"}, 
        number_plays: {$first:"$number_plays"}
    }}
], function(err,list){
    console.log(list);
    res.end();
});

由于您已经对number_plays进行了降级，因此将使用每个oid组文档中具有最高number_plays的值.

Because you've sorted on number_plays descending in the preceding stage of the pipeline, this will take the values from each oid group's doc with the highest number_plays.

这篇关于猫鼬聚合:限制$ group中的记录数的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！