Update by id with mongoose doesn't find the document - node.js

Question

Found the problem while writing this.
As it may help others not searching in the right place like me, you will find the answer below.
I can't seem to make a mongoose Model.update() request work, when a Model.findOne() with the same condition does.
To illustrate, here is my code:
Schema:
var GeolocSchema = new mongoose.Schema({
id: {
type: String,
required: true,
unique: true
},
count: Number
});
An express test route showing the seemingly broken update attempt:
router.get('/test', function(req, res, next) {
var doc = {
id: '50_633-20_059',
count: 3,
density: 1
};
var promise = Geoloc.update({
id: doc.id
}, {
$set: {
density: doc.density
}
}).exec();
promise.then((result, err) => {
if (err) return next(err, null);
res.status(201).json(result);
});
});
Results in:
{
"ok": 0,
"n": 0,
"nModified": 0
}
An express test route showing the working findOne call:
router.get('/test2', function(req, res, next) {
var doc = {
id: '50_633-20_059',
count: 3,
density: 1
};
var promise = Geoloc.findOne({
id: doc.id
}).exec();
promise.then((result, err) => {
if (err) return next(err, null);
console.log('Update result: ', result);
res.status(200).json(result);
});
});
Results in:
{
"_id": "5885d33239f30034de9a38d0",
"id": "50_633-20_059",
"count": 3,
"__v": 0,
}

Answer 1

Credits to this unaccepted answer for putting me on the right track.
The culprit was my schema.
Interestingly enough (for a schema-less database), an update operation can't be made on non-existing fields: probably a fail safe to prevent a DB flood from an external source.
Note that it works as expected via the Mongo CLI.
Which means that the only necessary change in the code above was the following:
Schema:
var GeolocSchema = new mongoose.Schema({
id: {
type: String,
required: true,
unique: true
},
count: Number,
density: Number // This is mandatory!
});
More infos here.