mapReduce using node.js and mongoose

Question

我正在计算每个地区的学生人数。我有一个型号，看起来像

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，有什么建议吗？

类型错误：

Cannot read property 'out' of undefined
at Collection.mapReduce (C:***node_modulesmongodblibcollection.js:2961:21)
at NativeCollection.(anonymous function) [as mapReduce] (C:***node_modulesmongooselibdriversnode-mongodb-nativecollection.js:136:28) 

Answer 1

尝试直接在模型上调用mapReduce()方法，而不是在模型的collection属性上调用，因为该属性需要一个额外的对象作为out属性的参数：

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，该框架具有更好的性能，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，您可以运行以下aggregation管道来获得相同的结果：

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)
    });
};