_form.html.haml

= fieldset :legend => "Images:" do
  - @product.max_attachments_size.times do |i|
    %p
      - if a = @product.attachments[i]
        = file_field :name => "product[attachments][#{a.id}][data]", :label => a.filename
        = check_box :name => "product[attachments][#{a.id}][delete]", :label => "Delete?".t
      - else
        = file_field :name => "product[attachments][new][]", :label => "Image %d:".t(i+1)

attachable.rb

module Attachable
  def self.included(base)
    base.class_eval do
      validates_with_method :validate_attachments
    end
  end
  
  def attachments
    @attachments ||= attachment_class.all(:attachable_type => self.class, :attachable_id => self.id)
  end
  
  def validate_attachments
    attachments.each_with_index do |att, i|
      if att.valid?
        att.save
      else
        return [false, "Images are invalid".t]
      end
    end
    true
  end
  
  def attachments=(set)
    Merb.logger.d set
    attachments.each do |att|
      a = set[att.id.to_s]
      if a[:delete]
        att.destroy
      elsif !(data = a[:data]).empty?
        att.update_file(data)
      end
    end
    
    (max_attachments_size - attachments.size).times do |i|
      unless (file = set[:new][i]).blank?
        att = attachment_class.new :attachable_type => self.class,
                             :attachable_id => self.id,
                             :file => file
        attachments << att
      end
    end
  end
  
end

attachment.rb

require "ftools"

class Attachment
  include DataMapper::Resource
  
  attr_accessor :tempfile
  
  property :id, Serial
  property :filename, String
  property :content_type, String
  property :size, Integer
  property :attachable_id, Integer
  property :attachable_type, String
  
  def url
    "/uploads/#{id}/#{filename}"
  end
  
  before :destroy, :delete_files
  after :create, :copy_files
  
  def delete_files
    FileUtils.remove_dir("public/uploads/#{id}")
  end
  
  def update_file(file)
    orig_filename = filename
    self.file = file
    
    if valid?
      delete_files
      save
      copy_files
      true
    else
      self.filename = orig_filename
      false
    end
  end
  
  def file=(file)
    self.filename     = file[:filename]
    self.content_type = file[:content_type]
    self.size         = file[:size]
    self.tempfile     = file[:tempfile]
  end
  
  def copy_files
    File.makedirs("public/uploads/#{id}")
    FileUtils.mv(tempfile.path, "public/uploads/#{id}/#{filename}")
  end
end

product.rb

class Product
  include DataMapper::Resource
  include Attachable
  
  def max_attachments_size
    4
  end
  
  def attachment_class
    Image
  end