gistfile1.rb

class Foo

  has_one :image

  def process_stuff

    img = Image.first

    img.with_temp_file('/path/to/tmpfile') do |tf|
      tf.rotate
      tf.bounce
      tf.other_thing
      img.save(:file => tf)
    end

  end
end


class Image

  belongs_to :foo


  def with_temp_file(path, &block)
    # download content, put it into a local fle
    tf = open(path, 'wb') { |fh|
      fh.write(downloaded_content)
      yield
    }

    tf.delete
  end

end