Mongoose models' save() won't update empty array - arrays

Question

I have an array (bookedby) in a Mongoose model defined like this:
var mongoose = require('mongoose');
var Schema = mongoose.Schema;
var BarSchema = new Schema({
date: {
type: Date,
required: true
},
barid: {
type: String,
required: true
},
bookedby: {
type: [String],
required: true
},
});
module.exports = mongoose.model('Bar', BarSchema);
I update it with following function, called by a nodejs express router:
const Bars = require("../../models/bars");
const { getToday } = require('../../utils');
module.exports = function(req, res) {
const { barid } = req.body;
const { username } = req.user;
const date = getToday();
if( !barid ) return res.json({ success: false, error: 'Please specify parameter \'barid\'.'})
Bars.findOne({ barid, date }, function (err, bar) {
if (err) return next(err);
if (!bar || bar.bookedby.indexOf(username) === -1) return res.json({ error: `Bar is not booked yet.` });
// Someone booked the bar
const index = bar.bookedby.indexOf(username);
bar.bookedby.splice(index, 1);
bar.save(err => {
if (err) res.json({ error: `Error saving booking.` });
else res.json({ success: true });
});
});
};
Everything works fine, except when I remove the last item from the bookedby array. Then the save() function doesn't update the database. The last item remains there. I guess it has something to do with mongodb optimizing empty arrays, but how can I solve this?

Answer 1

According to the Mongoose FAQ:
http://mongoosejs.com/docs/faq.html
For version >= 3.2.0 you should use the array.set() syntax:
doc.array.set(3, 'changed');
doc.save();
If you are running a version less than 3.2.0, you must mark the array modified before saving:
doc.array[3] = 'changed';
doc.markModified('array');
doc.save();