How to do a atomic find and save using mongoose in Node.js

Question

How to do a atomic find and save using mongoose in Node.js

In my db, I have a "Thing", whose id is "53e5fec1bcb589c92f6f38dd".
I wanna update the count member in it everytime when doAdd is called.
Like the code below.
But since the find and save operation is separated, I cannot get the desired result..
Any best practice about this situation?
Thanks in advance!

var mongoose = require('mongoose');
mongoose.connect('mongodb://localhost/test');

var ThingSchema = new mongoose.Schema({
  count: Number,
});

var Thing = mongoose.model('Thing', ThingSchema);

var doAdd = function(id){
  Thing.findById(id,  function(err, thing){ 
    if(******){ 
      #perform some logic...
      thing.count++;
    }
    thing.save();
  });
};

var id = "53e5fec1bcb589c92f6f38dd";
doAdd(id);
doAdd(id);

node.js
mongoose

Answer 1

The problem is you are expecting to increase the count by 2 as you are calling doAdd twice. But here the code in doAdd is of asynchronous nature i.e the second doAdd call is called before the the first query and save process is completed. The results are unpredictable due to the asynchronous nature of the function. All mongoose queries and the save calls are asynchronous. So if you call

thing.save();
console.log('saved!!');

The console ouput appears before the document is actually saved because node doesn't wait for the save to be finished. So if you need any operation to be done after the document is saved, you would place it in the callback.

thing.save(function(err){
       if(!err){
         console.log('saved');
       }
 });

If you want to avoid callbacks have a look at async module.