aggregate by nodejs mongoose library @ mongodb

mongoose_aggregate

var express = require('express');
var router = express.Router();

var Task = require('../models/task');
var Auditlog = require('../models/auditlog');
var User = require('../models/user');
var auditprivileges = require('../common').audits;
var _ = require('underscore');

var mongoose = require('../common').mongoose;
var Schema = mongoose.Schema;

var TaskSchema = new Schema({
  taskname: { type: String, default: '' },
  project: { type: Schema.Types.ObjectId, ref: 'Project'},
  // 1*16: dev, 2*16: test, 4*16: project manager, 8*16: tech manager
  tasktype: { type: Number, default: 16 },
  date: { type: Date, default: Date.now },
  rdmtask:  { type: String, default: '' },
  worker: { type: Schema.Types.ObjectId, ref: 'User' },
  wholequality: { type: Number, min: 0, max: 100, default: 0 },
  process: { type: Number, min: 0, max: 100, default: 0 },
  docquality: { type: Number, min: 0, max: 100, default: 0 },
  satisfied: { type: Number, min: 0, max: 100, default: 0 },
  innovate: { type: Number, min: 0, max: 100, default: 0 },
  standard: { type: Number, min: 0, max: 100, default: 0 },
  shares: { type: Number, min: 0, max: 100, default: 0 },
  weight: { type: Number, min: 0.8, max: 1.2, default: 1 },
  // final score
  score: Number,
  remark: { type: String, default: '' },
  mixed: Schema.Types.Mixed
});

/**
 * get ranks
 *
 *  @return {[type]}       [description]
 */
router.get('/ranks', function(req, res, next) {
  Task.aggregate(
    // show both total and average score, ordered by average asc.
    { $group: { _id: '$worker', totalscore: { $sum: '$score' }, avgscore: { $avg: '$score'}}},
    { $project: { _id: 1, totalscore: 1, avgscore: 1 } },
    { $sort: { avgscore: -1 }},
    function(err, idscores) {
      if (err) return next(err);
      User.populate(idscores,
        { path: '_id', model: 'User', select: 'name'},
        function(err, userscores) {
          res.json(userscores);
        }
      );
  });
});