ERR_CONNECTION_RESET upon Backbone.save() to update model by Mongoose - node.js

Question

Please, tell me what I am doing wrong:
My backbone view creates and saves document in mongodb by mongoose and uses the data in the view allright upon the backbone save() and fetch() methods. But when i use Backbone model.save({option: 'modified'}); with route.put() on the backend, jquery fires ERR_CONNECTION_RESET. I tried lots of things i found in the net, but they did not work. Maybe i need to use ajax.Prefilter or something of this kind, but i do not know exactly what to do.
the piece of code in backbone view firing update is:
this.user.save({ options: 'modified' }, {
wait: true,
success: function(model, response) {
console.log('saved');
},
error: function(model, error) {
console.log(error);
}
});
in router index.js
router.put('/user/:id', require('./user').put);
it works because on GET route it works perfectly well.
in user.js
exports.put = function(req, res) {
var query = { _id: req.params.id };
User.update(query, { options: req.body.options }, function(){
res.send('ok');
});
};
I also experimented a lot with id or _id, and various ways of using mongoose update, like
User.findOne({ _id: req.params.id }, function (err, doc){
if (err) return console.log(err);
doc.options = req.body.options;
doc.save(function(err){
res.json(doc);
});
});
and others such like. It did not work. Here is the error (it must be the key to my problem, but i cannot figer it out)
Synchronous XMLHttpRequest on the main thread is deprecated because of
its detrimental effects to the end user's experience.
PUT http://localhost:3000/user/56349be42b19125405c2d66a
net::ERR_CONNECTION_RESET

Answer 1

It was silly: maximum req size exceeded. Cured by setting
app.use(bodyParser.json({limit: '16mb'}));
app.use(bodyParser.urlencoded({ limit: '16mb', extended: true }));