Mongoose with Bluebird promisifyAll - saveAsync on model object results in an Array as the resolved promise value

Question

I'm using bluebird's promisifyAll with mongoose. When I call saveAsync (the promisified version of save) on a model object, the resolved value of the completed promise is an array with two elements. The first is my saved model object, the second is the integer 1. Not sure what's going on here. Below is example code to reproduce the issue.

var mongoose = require("mongoose");

var Promise = require("bluebird");


Promise.promisifyAll(mongoose);


var PersonSchema = mongoose.Schema({
    'name': String
});

var Person = mongoose.model('Person', PersonSchema);

mongoose.connect('mongodb://localhost/testmongoose');


var person = new Person({ name: "Joe Smith "});

person.saveAsync()
.then(function(savedPerson) {
    //savedPerson will be an array.  
    //The first element is the saved instance of person
    //The second element is the number 1
    console.log(JSON.stringify(savedPerson));
})
.catch(function(err) {
    console.log("There was an error");
})

The response I get is

[{"__v":0,"name":"Joe Smith ","_id":"5412338e201a0e1af750cf6f"},1]

I was expecting just the first item in that array, as the mongoose model save() method returns a single object.

Any help would be greatly appreciated!

Answer 1

Warning: This behavior changes as of bluebird 3 - in bluebird 3 the default code in the question will work unless a special argument will be passed to promisifyAll.

---

The signature of .save's callback is:

 function (err, product, numberAffected)

Since this does not abide to the node callback convention of returning one value, bluebird converts the multiple valued response into an array. The number represents the number of items effected (1 if the document was found and updated in the DB).

You can get syntactic sugar with .spread:

person.saveAsync()
.spread(function(savedPerson, numAffected) {
    //savedPerson will be the person
    //you may omit the second argument if you don't care about it
    console.log(JSON.stringify(savedPerson));
})
.catch(function(err) {
    console.log("There was an error");
})

Answer 2

Why not just using mongoose's built-in promise support?

const mongoose = require('mongoose')
const Promise = require('bluebird')

mongoose.Promise = Promise
mongoose.connect('mongodb://localhost:27017/<db>')

const UserModel = require('./models/user')
const user = await UserModel.findOne({})
// ..

Read more about it: http://mongoosejs.com/docs/promises.html