Save image size for paperclip photo

photo.rb

class Photo < ActiveRecord::Base

  has_attached_file :image, :styles => { :thumb => ['150x150#', :png],
                                         :normal => ['660x660', :png] }, 
                            :default_style => :normal
  typed_serialize :sizes, Hash
  before_save :cache_image_sizes

  # Get size of this photo for specified style
  #
  # Returns a string of the schema "{Width}x{Height}"
  def size(style = image.default_style)
    style = style.to_sym
    size = self.sizes[style]
  end

  private

    def cache_image_sizes
      self.image.styles.each do |style_name, style|
        self.sizes[style_name] = Paperclip::Geometry.from_file( self.image.to_file(style_name) ).to_s
      end
      
      return true
    end

end

photos_helper.rb

module PhotosHelper

  def image_tag(source, options = {})
    unless source.is_a?(Photo)
      super(source, options)
    else
      options = options.symbolize_keys
      style = options.has_key?(:style) ? options.delete(:style).to_sym : source.image.default_style
      
      super(source.image.url(style), {:size => source.size(style)}.merge(options))
    end
  end
  
end