class ImageParts << ActiveRecord::Base
  include PartUploader[:file]
  belongs_to :image
end

class Image << ActiveRecord::Base
  include ImageUploader[:file]
  
  has_many :image_parts
  [...]
  
  def create_file_from_parts!
    file.create_from_concatenation image_parts.map(&:file), name: "#{id}.jpg"
    save!
  end