controller

  post :add do
    if (file = params[:file])
      image = Image.create(
        :name => 'abc',
        :gallery_id => 1
      )

      image.upload(file)
    end
  end

model

class Image
  include DataMapper::Resource

  belongs_to :gallery
  has 1, :user, :through => :gallery

  # property <name>, <type>
  property :id, Serial
  property :name, String
  property :filename, String
  property :created_on, DateTime

  def upload(file)
    # Get upload path and create it if needed
    file_dir = self.user.get_file_dir
    FileUtils.mkdir_p(file_dir)

    filename = file[:filename]
    tmpfile = file[:tempfile]

    #Upload is done here...

    @filename = filename
    self.save
  end
end