Unexpected value for 'this' in express.js instantiated controllers

'this' does not appear to refer to the instantiated budget controller object. Instead it seems to refer to the global object. Does anyone know why this is?

I've defined a budget model. Injected into the controller and I'm attempting to simply generate a random 6 char string when I hit /budgets in my app. Instead this.DEFAULT_SLUG_LENGTH is undefined and I can't figure out why.

This is a dumbed down test case illustrating the issue with 'this'. I have a similar problem when referencing the injected this.budget within another function to query the db based on the slug value.

//models/budget.js
var Schema = require('jugglingdb').Schema;
var schema = new Schema('postgres',{url:process.env.DATABASE_URL});

var Budget = schema.define('budgets',{
  total: Number,
  slug: String
});

module.exports = Budget;
====================

//controllers/budget.js
function BudgetController (budget) {
  this.budget = budget;
};

BudgetController.prototype.DEFAULT_SLUG_LENGTH = 6;

BudgetController.prototype.generateSlug = function (req,res) {
  var slug = "";
  var possible = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789";
  for (var i = 0; i < this.DEFAULT_SLUG_LENGTH; i++) {
    slug += possible.charAt(Math.floor(Math.random() * possible.length));
  }
  res.send(slug);
};

module.exports = BudgetController;
===================

//app.js
var express = require('express');
var app = express();

app.use(express.bodyParser());

// models
var Budget = require('./models/budget');

// controllers
var BudgetController = require('./controllers/budget');
var budgetCtrl = new BudgetController(Budget);

// routes
app.get('/budgets',budgetCtrl.generateSlug);

app.listen(process.env.PORT || 4730);

If I manually instantiate the model/controller in the node repl, the generateSlug method works fine. If I restructure my code so that the BudgetController is a function that returns an object {} with methods, that seems to work fine. Is there some issue with my use of prototype/new ?