Jasmine Sinon Backbone

IndexModule.js

define(
  [
    "backbone", "marionette", "jquery",
    "modules/app/app",
    "text!modules/index/templates/weather.html",
    "text!modules/index/templates/multiple.html",
    "text!modules/index/templates/loading.html"
  ],
  function(Backbone, Marionette, $, App, weatherTpl, multipleTpl, loadingTpl) {
    var module = App.module('Index');

    // Views and Model definitions....

    // This callback is executed when "Module.start()"
    module.addInitializer(function() {
      // Put loading view under App main region
      var loadingView = new module.LoadingView();
      App.mainRegion.show(loadingView);

      // Get weather
      var weatherModel = new module.WeatherModel({
        query : "SP/Sevilla"
      });

      var weatherView = new module.WeatherView({
        model : weatherModel
      });

      // How can I test this on my spec?
      weatherModel.fetch();
    });

    return module;
  }
);

spec.js

define(
  ["modules/index/index"],
  function(Index) {
    describe("Index module", function() {

      it("Should fetch WeatherModel on start", function() {
        // With this two spies I assert that "new WeatherModel" and "new WeatherView" are called at start of IndexModule
        var spyWeatherModel = sinon.spy(Index, "WeatherModel");
        var spyWeatherView = sinon.spy(Index, "WeatherView");

        // How do I assert that "weatherModel.fetch();" has been called on "Index.start()"? I can't spy on "fetch" because: "Attempted to wrap undefined property fetch as function"
        var spyWeatherModelFetch = sinon.spy(Index.WeatherModel, "fetch");

        Index.start();

        expect(spyWeatherModel).toHaveBeenCalled();
        expect(spyWeatherView).toHaveBeenCalled();
        expect(spyWeatherModelFetch).toHaveBeenCalled();
      });

    });
  }
);