Ember.js - can't get or set model property

I have an Ember.js model with fixtures that looks like this:

App.Category = DS.Model.extend({
    category: attr('string'),
    friendly: attr('string'),
    iconUrl: attr('string'),
    isPrimary: attr('bool'),
    isSecondary: attr('bool'),
    isTertiaryOne: attr('bool'),
    isTertiaryTwo: attr('bool')
});

App.Category.reopenClass({
    FIXTURES: [
        {
            id: 1,
            category: 'recommended',
            friendly: 'recommended for you',
            iconUrl: 'image1.png',
            isPrimary: true,
            isSecondary: false,
            isTertiaryOne: false,
            isTertiaryTwo: false
        },
        {
            id: 2,
            category: 'recent',
            friendly: 'recently viewed',
            iconUrl: 'image2.png',
            isPrimary: false,
            isSecondary: true,
            isTertiaryOne: false,
            isTertiaryTwo: false
        }
    ]
});

All I want to do is retrieve a property value from a specific model, and set it to a new value in an action in my controller:

App.CategoryController = Ember.ArrayController.extend({    
    actions: {
        tileClick: function (selectedCategory) {
            var cat = this.store.find('category', { category: selectedCategory });
            console.log(cat.get('isPrimary'));
            cat.set('isPrimary', true);
        }
    }
});

The Emberjs website guides say that all I have to do to set a value is :

var tyrion = this.store.find('person', 1);
// ...after the record has loaded
tyrion.set('firstName', "Yollo");

but it just isn't working.

The variable 'cat' exists, and if I drill down deep enough into the object in the console, I can see the properties I want, so I know the correct model is being selected.