gistfile1.rb

= form_for @gallery, :html => { :multipart => true }, :remote => true do |f|
  %ol.dialog.form
    %li.text
      = f.label :title
      = f.text_field :title
    %li.select
      = f.label :parent_id, 'Parent'
      = f.select :parent_id , @galleries.collect {|g| [g.title, g.id]}, { :include_blank => 'Pick a parent' }
    %li.file
      - f.fields_for :image do |image|
        = image.label :asset, 'Thumbnail'
        = image.file_field :asset
    %li.submit
      = f.submit
  #form_wait

gistfile2.rb

class PhotoGalleriesController < ApplicationController
  
  respond_to :html, :json, :js
  
  ...

  def new
    @galleries = PhotoGallery.current_clients_galleries(current_client.id)
    @gallery   = PhotoGallery.new
    @gallery.build_image
    
    @form = render_to_string :partial => 'form'
    
    respond_with(@gallery)
  end

gistfile3.rb

class PhotoGallery < ActiveRecord::Base

  # Poster image for gallery
  has_one     :image, 
              :conditions  => { :table_name => :galleries },
              :foreign_key => "table_id"
  
  accepts_nested_attributes_for :image, :reject_if => proc { |attributes| attributes['asset'].blank? }

  ...

gistfile4.rb

class Image < ActiveRecord::Base
  
  ...

  belongs_to :photo_gallery, :foreign_key => 'table_id', :inverse_of => :image
  
  
  has_attached_file :asset,
                    :storage => :s3,
                    :s3_credentials => "#{RAILS_ROOT}/config/s3.yml",
                    :path => "/media/:class/:attachment/:style.:filename",
                    :bucket => '******'

   ...