Ive coded out a simple query of $sum but it doesnt seem to be showing me the output i want. It is supposed to show the average air_temperature for all the machine_unit.

var express = require('express');
var app = express();
var mongojs = require('mongojs');
var db = mongojs('meibanlist', ['meibanlist']);
var bodyParser = require('body-parser');

app.use(express.static(__dirname + '/public'));
app.use(bodyParser.json());

app.get('/meibanlist', function (req, res) {
  console.log('I received a GET request');

  db.meibanlist.find(function (err, docs) {
    console.log(docs);
    res.json(docs);
  });
});

app.post('/meibanlist', function (req, res) {
  console.log(req.body);
  db.meibanlist.insert(req.body, function(err, doc) {
    res.json(doc);
  });
});

app.delete('/meibanlist/:id', function (req, res) {
  var id = req.params.id;
  console.log(id);
  db.meibanlist.remove({_id: mongojs.ObjectId(id)}, function (err, doc) {
    res.json(doc);
  });
});

app.get('/meibanlist/:id', function (req, res) {
  var id = req.params.id;
  console.log(id);
  db.meibanlist.findOne({_id: mongojs.ObjectId(id)}, function (err, doc) {
    res.json(doc);
  });
});

app.put('/meibanlist/:id', function (req, res) {
  var id = req.params.id;
  console.log(req.body.machine_unit);
  db.meibanlist.findAndModify({
    query: {_id: mongojs.ObjectId(id)},
    update: {$set: {machine_unit: req.body.machine_unit, air_temperature: req.body.air_temperature, water_temperature: req.body.water_temperature, heat_temperature: req.body.heat_temperature, room_temperature: req.body.room_temperature, date: req.body.date, time: req.body.time}},
    new: true}, function (err, doc) {
      res.json(doc);
    }
  );
});

///this is where it all went wrong

db.meibanlist.aggregate([{$project:{ "Average": { $avg: ["$air_temperature"] }}}], function(err, meibanlist){
  if (err || !meibanlist) console.log ("Record not found");
  else meibanlist.forEach (function(machine_unit){
    console.log(machine_unit);

  });
});


app.listen(3000);
console.log("Server running on port 3000");