Dynamic computed properties in Ember.JS deprecated?

I am trying to make an ember application. I have a computed property and the controller looks like this:

// The Controller

Todos.Controller = Ember.Controller.create({

    // ** SNIP ** //

    countCompleted: function()
    {
        return this.get('todos').filterProperty('completed', true).length
    }.property(),
});

// The View

{{Todos.Controller.countCompleted.property}} Items Left

Now the tutorial I'm following is using an older version of Ember.JS. I've fixed every error but this:

Uncaught Error: assertion failed: Ember.Object.create no longer supports defining computed properties.

What's the alternative way to do this?