How to implement a subtotal of all orders?

The following, renders: Subtotal: P[object Object]

It seems that subtotal is returning the promise object instead of th sum of all orders. VARIATION 2 also returns a promise.

How should I go about calculating the subtotal of all products?

  // app/controllers/application.js
  import Ember from 'ember';

  export default Ember.ObjectController.extend({    
    subtotal: function() {
      // VARIATION 1:
      var productCollectionPromises = this.get('orders').getEach('product');
      var sum = 0;

      return Ember.RSVP.all(productCollectionPromises).then(function(productCollections){
        productCollections.forEach(function(product){
          sum += product.get('amountInCents');
        });

        return sum;
      });

      // VARIATION 2:
      // return this.get('orders').mapBy('product').reduce(function(previousValue, product) {
      //   return previousValue + product.get('amountInCents');
      // }, 0) / 100;
    }.property('orders.@each.total'),
  });

  // app/templates/application.hbs
  <br /><strong>Subtotal:</strong> ${{subtotal}}