gen md5 hash

gistfile1.rb

  # note the caller ensures that "prefix" consists of something
  # uniquely identifying the user/brochure combination
  def images_key(images, prefix = "")
    key = "#{prefix} "
    images.each do |im| 
      key += "#{im.file.size} #{im.file_updated_at}"
    end
    Digest::MD5.new.hexdigest(key)
  end