gistfile1.rb

require 'ftools'
class Appartment
  include DataMapper::Resource

  attr_accessor :app_images

  property :id, Serial
  property :name, String, :nullable => false, :unique => true
  property :bedrooms, Integer, :nullable => false
  property :bathrooms, Integer, :nullable => false
  property :beds, Integer, :nullable => false
  property :description, Text, :nullable => false

  has n, :images

  after :create, :create_images

  private

  # Loops over all the images and runs create_image on them
  def create_images
    self.app_images.each { |image| create_image(image) }
  end

  # Creates a image row in the associated image table for the image,
  # and moves the image into the right folder
  def create_image(image)
    appartment_image = Image.create(:appartment_id => self.id,
                                    :filename => image[:filename],
                                    :content_type => image[:content_type],
                                    :size => image[:size]
                                    )
    File.makedirs("public/uploads/#{self.id}")
    File.move(image[:tempfile].path, "public/uploads/#{self.id}/#{image[:filename]}")
  end

end

gistfile2.rb

class Image
  include DataMapper::Resource

  property :id, Serial
  property :filename, String, :nullable => false
  property :content_type, String, :nullable => false
  property :size, Integer, :nullable => false
  property :appartment_id, Integer, :nullable => false

  belongs_to :appartment

  def url
    "/uploads/#{self.attachable_id}/#{self.filename}"
  end

end