Mongoose save callback doesn't fire

I'm new to mongoose and I'm having a hard time finding the issue within my code. I'm building a REST server using Sails.js and Mongoose. I have a node module (e.g. "sails-mongoose") for exporting mongoose, where I also connect to my database:

var mongoose = require('mongoose');
mongoose.connect('mongodb://@localhost:27017/fooria');
module.exports = mongoose;

And in my model.js:

var adapter = require('sails-mongoose');
var schema = new adapter.Schema({
    firstname: {
        type: String,
        required: true,
        trim: true
    }
});
module.exports = {
    schema: schema,
    model: adapter.model('Collection', schema)
}

In my controller's create method I have:

create: function(req, res, next) {
    var userData = {firstname: 'Test'};
    var users = new Users.model(userData);
    users.save(function(err, data){   
       if (err) return res.json(err, 400);
       res.json(data, 201);
   });
}

When running create method, the entry is saved to the Mongodb collection but the callback is never reached. Can someone please help me on this track, as I found similar questions but none helped me though. Thanks!