mongodb（map-reduce）

var user    = new Schema({    
    username:{type:String}, 
    
    password:{type:String},
})

exports.test = function(){    
    var user = require(‘../dao/user‘);
    var model = user.model;
    
    var option = {};
    //输出为_id:name, value:1
    option.map = function () { emit(this.username, 1) }
    //将键值相同的传给同一个reduce函数，此时k为_id:name, vals 为value的数组
    //当不存在多个键值对时，该reduce函数不会被调用
    option.reduce = function (k, vals) {
        return vals.length}
    model.mapReduce(o, function (err, results) {
      console.log(results)
    });
}

//数据库中存有三条记录，一条用户名为hello，两条用户名为test
[ { _id: ‘hello‘, value: 1 }, { _id: ‘test‘, value: 2 } ]