how to include external models in mongoose?

i have a problem when i want to include my models with mongoose in nodejs, i'm create a models with schema like this

var mongoose = require('mongoose')
  , Schema = mongoose.Schema;

var Users = new Schema({
    idUser  : {type:String},
    username: {type:String}
});

// middleware
Users.pre('save', function (next,done) {
  notify(this.get('email') + done);
  // something goes wrong
  next(new Error('something went wrong'));
});

//registered on mongoose models
mongoose.model("Users",Users);

and i save in folde models/schema.js but i don't know how to call that file in app.js , when i'm try using this code

var mongoose = require('mongoose')
  , models = require('./models/schema.js');

//mongoose configurationfor database;
var db = mongoose.connect("mongodb://localhost/vynchat");
var users = mongoose.model("Users");
users.save();

i have error when i try to start sudo node app.js

throw e; // process.nextTick error, or 'error' event on first tick
              ^
TypeError: Object function model() {
    Model.apply(this, arguments);
  } has no method 'save'
    at Object.<anonymous> (/opt/development/vynapp/app.js:18:7)
    at Module._compile (module.js:441:26)
    at Object..js (module.js:459:10)
    at Module.load (module.js:348:31)
    at Function._load (module.js:308:12)
    at Array.0 (module.js:479:10)
    at EventEmitter._tickCallback (node.js:192:40)

the error model() has no method save... how i can fix this..?