listing.rb

class Listing
  markdown :title, :description
end

markdown.rb

class MarkdownAttribute
  def self.load value
    return nil if value.nil? # or something
    
    MarkdownAttribute.new(value)
  end
  
  def self.dump obj
    return nil if obj.nil? # or something
    obj.markdown
  end
  
  def initialize value
    @raw = value
  end
  
  def to_s
    Redcarpet::Markdown.new(RemoveMarkdownRenderer).render(markdown)
  end
  
  def markdown
    @raw.to_s
  end
  
  def html
    Redcarpet::Markdown.new(Redcarpet::Render::HTML).render(markdown).safe_html
  end
end

markdown_concern.rb

module MarkdownHandler
  extend ActiveSupport::Concern

  module ClassMethods
    def markdown( *attributes_with_markdown )
      attributes_with_markdown.each do |attribute|
        serialize attribute, MarkdownAttribute
    end
  end
end