## model

class Asset < ActiveRecord::Base

  has_attachment :storage => :file_system, 
    :thumbnails => { :polaroid => '193x193', :fullscreen => '800>', :large => "300>", :thumb => '120>', :tiny => '50>' }, 
    :max_size => 5.megabytes,
    :path_prefix => "public/image_assets"
  validates_as_attachment
  
  def regenerate_thumbnail
    ActiveRecord::Base.record_timestamps = false
    begin
      self.temp_data = temp_data
      save!
    ensure
      ActiveRecord::Base.record_timestamps = true
    end
  end

end

## migration

class RegenerateThumbs < ActiveRecord::Migration
  def self.up
    Asset.find(:all, :conditions => 'parent_id is null').each &:regenerate_thumbnail
  end

  def self.down
  # poo
  end
end