Rails: ViewModel

class ViewModel
  class Attribute
    attr_reader :name
    attr_accessor :model_value, :view_value

    def initialize(name, model_value = :_undefined, view_value = :_undefined)
      @name = name
      @model_value = model_value
      @view_value  = view_value
    end

    def has_model_value?
      @model_value != :_undefined
    end

    def has_view_value?
      @view_value != :_undefined
    end
  end

  attr_reader :attributes

  def initialize(options = {})
    @attributes = HashWithIndifferentAccess.new

    self.model_attributes = options[:model_attributes] if options[:model_attributes]
    self.view_attributes  = options[:view_attributes]  if options[:view_attributes]
  end

  def model_attributes=(attrs)
    attrs.each do |name, value|
      (@attributes[name] ||= Attribute.new(name)).model_value = value
    end
  end

  def model_attributes
    map_hash_values(@attributes) { |name| model_value(name) }
  end

  def view_attributes=(attrs)
    attrs.each do |name, value|
      (@attributes[name] ||= Attribute.new(name)).view_value = value
    end
  end

  def view_attributes
    map_hash_values(@attributes) { |name| view_value(name) }
  end

  def model_value(name)
    attr = @attributes.fetch(name)

    attr.has_model_value? ? attr.model_value : transform_view_value(name, attr.view_value)
  end

  def view_value(name)
    attr = @attributes.fetch(name)

    attr.has_view_value? ? attr.view_value : transform_model_value(name, attr.model_value)
  end


  # override points
  def transform_model_value(name, model_value)
    model_value
  end

  def transform_view_value(name, view_value)
    view_value
  end

  private

  def map_hash_values(hash)
    hash.inject(HashWithIndifferentAccess.new) do |ret, (k, v)|
      ret.update k => yield(k, v)
    end
  end

end