EmberJS - hasMany Fixtures Model Empty Value

I have 3 different fixture models, as shown below.

var Room = DS.Model.extend({
       title: DS.attr('string'),
       categories: DS.hasMany('Category', { async: true }),
       isSelected: DS.attr('boolean')
});
var Category = DS.Model.extend({
    title: DS.attr('string'),
    room: DS.belongsTo('Room', {async: true }),
    materials: DS.hasMany('Material', { async: true }),
    isSelected: DS.attr('boolean')
});
var Material = DS.Model.extend({
    title: DS.attr('string'),
    category: DS.belongsTo('Category', {async: true} ),
    isSelected: DS.attr('boolean')
});

I find when I try to view the contents inside the Materials model it is blank. In my controller I expose the materials by doing this:

currentMaterials: function() {
  var room = this.filterBy('isSelected', true).get('firstObject');
  var categories = room.get('categories');
  var selectedCategory = categories.get('firstObject');
  var material = selectedCategory.get('materials');
  return material;

}.property('@each.isSelected')

However when I try to access currentMaterials the value is null. I am ONLY able to access its values if I first access the Rooms/Categories using a {{#each} loop. Oddly once I do the {{#each}} I am then able to access the values in currentMaterials.

Does anyone understand why?