Downsample attachment_fu images

gistfile1.rb

class Image << ActiveRecord::Base
  has_attachment :storage => :file_system,
                 :content_type => :image,
                 :processor => 'rmagick',
                 # ...

  named_scope :large, :conditions => "size > #{1.megabytes}"

  DEFAULT_QUALITY = 60

  # ...

  def downsample!(quality=DEFAULT_QUALITY)
    image = Magick::ImageList.new(self.absolute_filename)
    image.write("jpeg:#{self.absolute_filename}") {self.quality = quality}
  end

  def absolute_filename
    File.join(Rails.root, 'public', self.public_filename)
  end
end