QUnit, Sinon.js & Backbone unit test frustration: sinon spy appears to fail to detect Backbone Model event callbacks

Question

In the following unit test code:

TestModel = Backbone.Model.extend({
    defaults: {
        'selection': null
    },
    initialize: function() {
      this.on('change:selection', this.doSomething);
    },
    doSomething: function() {
        console.log("Something has been done.");
    }
});

module("Test", {
    setup: function() {
        this.testModel = new TestModel();
    }
});

test("intra-model event bindings", function() {
    this.spy(this.testModel, 'doSomething');
    ok(!this.testModel.doSomething.called);
    this.testModel.doSomething();
    ok(this.testModel.doSomething.calledOnce);
    this.testModel.set('selection','something new');
    ok(this.testModel.doSomething.calledTwice); //this test should past, but fails.  Console shows two "Something has been done" logs.
});

The third ok fails, even though the function was effectively called from the backbone event binding, as demo'd by the console.

This is very frustrating and has shaken my confidence on whether sinon.js is suitable for testing my backbone app. Am I doing something wrong, or is this a problem with how sinon detects whether something has been called? Is there a workaround?

EDIT: Here's a solution to my specific example, based on the monkey patch method of the accepted answer. While its a few lines of extra setup code in the test itself, (I don't need the module function any more) it gets the job done. Thanks, mu is too short

test("intra-model event bindings", function() {
    var that = this;
    var init = TestModel.prototype.initialize;
    TestModel.prototype.initialize = function() {
        that.spy(this, 'doSomething');
        init.call(this);
    };

    this.testModel = new TestModel();
    . . . // tests pass!
}); 

Answer 1

Calling this.spy(this.testModel, 'doSomething') replaces the testModel.doSomething method with a new wrapper method:

var spy = sinon.spy(object, "method");

Creates a spy for object.method and replaces the original method with the spy.

So this.spy(this.testModel, 'doSomething') is effectively doing something like this:

var m = this.testModel.doSomething;
this.testModel.doSomething = function() {
    // Spying stuff goes here...
    return m.apply(this, arguments);
};

This means that testModel.doSomething is a different function when you bind the event handler in initialize:

this.bind('change:selection', this.doSomething);

than it is after you've attached your spying. The Backbone event dispatcher will call the original doSomething method but that one doesn't have the Sinon instrumentation. When you call doSomething manually, you're calling the new function that spy added and that one does have the Sinon instrumentation.

If you want to use Sinon to test your Backbone events, then you'll have to arrange to have the Sinon spy call applied to the model before you bind any event handlers and that probably means hooking into initialize.

Maybe you could monkey-patch your model's initialize to add the necessary spy calls before it binds any event handlers:

var init = Model.prototype.initialize;
Model.prototype.initialize = function() {
    // Set up the Spy stuff...
    init.apply(this, arguments);
};

Demo: http://jsfiddle.net/ambiguous/C4fnX/1/

You could also try subclassing your model with something like:

var Model = Backbone.Model.extend({});
var TestModel = Model.extend({
    initialize: function() {
        // Set up the Spy stuff...
        Model.prototype.initialize.apply(this, arguments);
    }
});

And then use TestModel instead of Model, this would give you an instrumented version of Model in TestModel without having to include a bunch of test-specific code inside your normal production-ready Model. The downside is that anything else that uses Model would need to be subclassed/patched/... to use TestModel instead.

Demo: http://jsfiddle.net/ambiguous/yH3FE/1/

You might be able to get around the TestModel problem with:

var OriginalModel = Model;
Model = Model.extend({
    initialize: function() {
        // Set up the Spy stuff...
        OriginalModel.prototype.initialize.apply(this, arguments);
    }
});

but you'd have to get the ordering right to make sure that everyone used the new Model rather than the old one.

Demo: http://jsfiddle.net/ambiguous/u3vgF/1/