How to Unit Test two dependant models in Ember.js?

I'm developing a small application in Ember.js and I'm having some troubles trying to unit test it. I have the following code:

// app/models/book.js
export default DS.Model.extend({
  name: DS.attr('string'),
  author: DS.attr('string'),
  rating: DS.attr('number'),
  category: DS.belongsTo('category', {
    async: true
  })
});

// app/models/category.js
export default DS.Model.extend({
  name: DS.attr('string'),
  bestRated: function () {
    return this.get('books').sortBy('rating').reverse();
  }.property('books.@each'),
  books: DS.hasMany('modules', {
    async: true
  })
});

// tests/unit/models/category-test.js
moduleForModel('category', 'Category Model', {
  needs: ['model:book']
});
test('category bestRated ... ok', function (assert) {
  var category = this.subject();
  // I want to populate category.modules with test modules, 
  // but I don't know how to do it.
  // If I try to use Ember.Object or Ember.ObjectProxy,
  // the test won't pass saying that cannot populate 
  // with undefined, only book is allowed
});

How can I create test books to test category?