Batman.js calcuated properties over collections

calculated_properties.coffee

class App.Sale extends Batman.Model 
  @hasMany 'sale_items'
  
  @accessor 'total_price', ->
    memo = 0
    # adds 'sale_items' and each item as a source
    @get('sale_items').forEach (item) ->
      memo += item.get('subtotal')
    memo
      
class App.SaleItem extends Batman.Model
  # adds quantity and price as sources 
  @accessor 'subtotal', -> @get('quantity') * @get('price')