What is the correct way of using Bluebird for Mongoose promises?

Question

I've been reading documentaion and articles and everyone seems to describe a different way about using Mongoose and Bluebird together. Even the official Mongoose documentation says something and Bluebird documentaion says another thing.

According to Mongoose:

mongoose.Promise = require('bluebird');

According to Bluebird:

var Promise = require("bluebird");
Promise.promisifyAll(require("mongoose"));

So to my understanding, if you pick the Mongoose way a sample query would be like:

User.findById('someId')
    .then(function(){
        // do stuff
    })
    .catch(function(err){
        // handle error
    })

But also in Mongoose docs it says that:

Mongoose queries are not promises. However, they do have a .then() function for yield and async/await. If you need a fully-fledged promise, use the .exec() function.

So in this case, is .then above a promise or not?

If you go with Bluebird way:

User.findById('someId')
    .execAsync()
    .then(function(){
        // do stuff
    })
    .catch(function(err){
        // handle error
    })

Or maybe even skip execAsync() and start with findByIdAsync instead.

Really confused with different documentaion. I'd appreciate if someone can shed some light into this.

Answer 1

Choose the Mongoose way:

mongoose.Promise = require('bluebird');

That's because Mongoose already supports promises (besides also accepting callbacks); the above code just replaces Mongoose's own promise library (mpromise) by Bluebird (which is probably faster, better tested, and has more utility functions available).

Bluebird's promisify*() is meant to allow code that doesn't already use promises (purely callback-based functions) to return promises.

Answer 2

From Bluebird doc

Promise.promisifyAll(
    Object target,
    [Object {
        suffix: String="Async",
        multiArgs: boolean=false,
        filter: boolean function(String name, function func, Object target, boolean passesDefaultFilter),
        promisifier: function(function originalFunction, function defaultPromisifier)
    } options] ) -> Object

as you can see, by default, promisifyAll add suffix 'Asyns', so if you are using this way of promisification:

var Promise = require("bluebird");
Promise.promisifyAll(require("mongoose"));

the async version of User.findById will be User.findByIdAsync

what about mongoose.Promise

then you use promise library like

mongoose.Promise = require('bluebird');

built-in promise mechanism replaced by library: query.exec().constructor replaced by require('bluebird') so instead .exec() for return promise, you can use bluebird probabilities directly for mongoose queries like

User.findOne({}).then(function(user){
    // ..
})