node使用 mongoose聚合 group

var mongoose = require('mongoose');
mongoose.connect("mongodb://localhost:27017/test", function(err) {
    if(err){
        console.log('连接失败');
    }else{
        console.log('连接成功');

    }
});

var schema = new mongoose.Schema({ num:Number, name: String, size: String},{ timestamps: true });
var MyModel = mongoose.model('MyModel', schema);



var group = {
   key: {"name":true},
   cond: {},
   reduce: function(doc, out) {
      out.count++;
      out.total += doc.size;
   },
   initial: {
       total: 0,
       count: 0
   },
   finalize: function(out) {
       out.avg = out.total / out.count;
   }
};


 //这里参数数量一定要保持一致，否则会报错
 MyModel.collection.group(
	group.key, 
	group.cond, 
	group.initial, 
	group.reduce, 
	group.finalize, 
	true, 
	function(err, reslut) {
 		console.log("result====",reslut)
	})

　　使用aggregate

 MyModel.aggregate([
  	 {$match: {}},
    {$group: {_id: "$name", total: {$sum: "$num"}}}
  ]).exec(function(err,reslut){
     console.log(result)
  })