使用node.js和mongoose的mapReduce

Rahul Ganguly 发表于 Dev

拉胡尔·甘古利（Rahul Ganguly）

我正在尝试计算每个地区的学生人数。我有一个看起来像的模型

var mongoose = require('mongoose');
var schema = mongoose.Schema;
var studentSchema = new mongoose.Schema(
{
 "name":String,
 "address" :{
     "locality":String
  }
});
module.exports = mongoose.model('Student', studentSchema);

然后我有一些Node.js代码

var Student = require('../../../models/Student');
module.exports.getStudentsBasedOnLocality = function(){
var o = {};
o.map = function () {
    emit(Student.address.locality, 1)
}
o.reduce = function (k, vals) {
    return vals.length
}

Student.collection.mapReduce(o, function (err, results) {
    if(err) throw err;
    console.log(results)
})
};

我得到的错误是。关于我可能做错了什么的提示？

TypeError：

Cannot read property 'out' of undefined
at Collection.mapReduce (C:\***\node_modules\mongodb\lib\collection.js:2961:21)
at NativeCollection.(anonymous function) [as mapReduce] (C:\***\node_modules\mongoose\lib\drivers\node-mongodb-native\collection.js:136:28)

克列丹

尝试mapReduce()直接在模型上而不是在模型的collection属性上调用该方法，该方法需要使用out属性作为参数的额外对象：

var Student = require('../../../models/Student');
module.exports.getStudentsBasedOnLocality = function(){
    var o = {},
        self = this;
    o.map = function () {
        emit(this.address.locality, 1)
    };
    o.reduce = function (k, vals) {
        return vals.length
    };

    Student.mapReduce(o, function (err, results) {
        if(err) throw err;
        console.log(results)
    });
};

另一种选择是使用聚合框架，因为聚合在服务器（C ++）中本地运行，而mapReduce生成单独的javascript线程来运行JavaScript代码，因此它具有更好的性能。因此，您可以运行以下aggregation管道以实现相同的结果：

var Student = require('../../../models/Student');
module.exports.getStudentsBasedOnLocality = function(){
    var pipeline = [
        {
            "$group": {
                "_id": "$address.locality",
                "count": { "$sum": 1 }
            }
        }
    ];

    Student.aggregate(pipeline, function (err, results) {
        if(err) throw err;
        console.log(results)
    });
};