mapReduce 使用 node.js 和猫鼬

Question

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。关于我可能做错的任何提示？

类型错误：

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) 

Answer 1

尝试直接在模型上调用 mapReduce() 方法，而不是在模型的集合属性上调用，因为它需要一个额外的对象作为带有 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 aggregation framework，它具有更好的性能，因为聚合在服务器 (C++) 中本机运行，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)
    });
};