Mongoose reacting differently to presence of callbacks - node.js

Question

I was debugging a chunk of my Mongoose queries that I wanted to execute one after another to avoid inconsistency. After lot of trial, errors and tests, I finally stumbled upon a solution that seemed to work.
In one query, I include a query and in another, I skip including the callback function. The callback I am referring to is seen in the syntax of a Model.findByIdAndUpdate statement.
Model.findByIdAndUpdate(id, [update], [options], [***callback***])
The one which has a callback function executes perfectly and the one which doesn't, doesn't execute. It's encapsulated in a function which returns a mongoose promsise.
Here is the first query with the schema in Robomongo which adds an 'organiser' successfully:
function addUserToHackathonOrganisers(userId, hackathonId) {
return Hackathon.findOneAndUpdate(
{hackathonId: hackathonId},
{$addToSet: {organisers: userId.toString()}},
{new: true}
);
}
Notice organisers Array[0] which indicates no update.
Now, here is the query that works (with the callback in). In the attached image, you can see the organisers Array[1] which indicates that its updated.
function addUserToHackathonOrganisers(userId, hackathonId) {
return Hackathon.findOneAndUpdate(
{hackathonId: hackathonId},
{$addToSet: {organisers: userId.toString()}},
{new: true},
function (err) {
if (err) {
console.log(err);
}
}
);
}
I'm clueless why this happens. Any ideas?

Answer 1

It is because the findOneAndUpdate() method without a callback option returns a Query, it does not execute but the one with a callback executes hence why you see an update with the callback and none without.
From the docs:
A.findOneAndUpdate(conditions, update, options, callback) // executes
A.findOneAndUpdate(conditions, update, options) // returns Query
A.findOneAndUpdate(conditions, update, callback) // executes
A.findOneAndUpdate(conditions, update) // returns Query
A.findOneAndUpdate() // returns Query
If you had written the function as
function addUserToHackathonOrganisers(userId, hackathonId) {
return Hackathon.findOneAndUpdate(
{hackathonId: hackathonId},
{$addToSet: {organisers: userId.toString()}},
{new: true}
).exec(); // chaining exec() to a Query returns a Promise
}
then when you call it, it executes and returns a promise, e.g.
var promise = addUserToHackathonOrganisers(userId, hackathonId);
promise.then(function(addedUser){
console.log(addedUser); // shows the updated user
});
UPDATE
To address your follow-up question in the comments, the callback in the promise's then() method
promise.then(function(addedUser){
console.log(addedUser); // shows the updated user
});
is just the same as the callback in the findOneAndUpdate() option except that instead of two arguments, the promise handles the arguments separately with the error argument caught in the promise's catch() method:
promise.then(function(addedUser){
console.log(addedUser); // shows the updated user
})
.catch(function(err){
console.log(err);
});
For more details, refer to the docs here.