Backbone.Model extension that calculates dependent attributes (computed properties)

computed-attributes-model.coffee

class ComputedAttributesModel extends Backbone.Model
  initialize: ->
    super
    return if _.isUndefined(@computed)
    for attr, dependencies of @computed           
      @bind "change:#{attr}", =>
        param = {}
        param["_#{attr}"] = @[attr].call @
        @set(param)

      _(dependencies).each (dependencyAttr) =>
        @bind "change:#{dependencyAttr}", =>
          @trigger "change:#{attr}"
        @trigger "change:#{attr}" if @has(dependencyAttr)
  get: (attr) ->
    attr = "_#{attr}" if @computed?.hasOwnProperty(attr)
    super attr