Ember 2, filter relationship models (hasMany, belongsTo) and calculte computed property based on relationships.

These are my files:

Models

app/models/basket.js:

export default DS.Model.extend({
  name: DS.attr('string'),
  house: DS.belongsTo('house', { async: true }),
  boxes: DS.hasMany('box', { async: true })
});

app/models/box.js:

export default DS.Model.extend({
  qty: DS.attr('number'),
  basket: DS.belongsTo('basket'),
  cartLines: DS.hasMany('cart-line', { async: true })
});

app/models/cart-line.js:

export default DS.Model.extend({
  qty: DS.attr('number'),
  box: DS.belongsTo('box'),
  product: DS.belongsTo('product')
});

app/models/product.js:

export default DS.Model.extend({
  name: DS.attr('string'),
  price: DS.attr('number')
});

Routes

app/routes/basket.js:

export default Ember.Route.extend({
  model(params) {
    return Ember.RSVP.hash({
      basket: this.store.findRecord('basket', params.basket_id),
      boxes: this.store.findAll('box'),
      products: this.store.findAll('product')
    });
  },
  setupController(controller, models) {
    controller.setProperties(models);
    }
});

Controllers

app/controllers/basket.js:

export default Ember.Controller.extend({
  subTotal: Ember.computed('boxes.@each.cartLines', function () {
    return this.products.reduce((price, product) => {
      var total = price + product.get('price');
      return total;
    }, 0);
  })
});

Questions:

I'm newbie, so I'm studying and makings mistakes. Sorry.

1) Which is the best Ember way to filter relationships when I first enter in route? For example now I load every box in my app whith boxes: this.store.findAll('box'). I need a way to not load all the box in my webapp, just the one in basket. I need the "query with filter" directly from a backend?

2) Which is the best Ember way for calculate subTotal? Now Ember gives me the subTotal of all products! Not just the total of my products in basket. Why this? I studied this: "http://ift.tt/2dV844x" but nothing. This code in controller gives me an error:

subTotal: computed('boxes.@each.cartLines', function() {
  return DS.PromiseArray.create({
    promise: this.get('boxes').then(boxes => {
      return boxes.filter(i => i.get('cart-line'));
    })
  });
})

and I know this is not logically correct, but just now it gives me: "basket.js:8 Uncaught TypeError: this.get(...).then is not a function". Why this?

3) Am I right with that relationship encapsulation?