get '/:post_id' do
  last_modification_of_post = Blog::Post[ params[:post_id], {:only => ['id', 'updated_on']} ]
  last_modified( last_modification_of_post.updated_on ) if last_modification_of_post
  @post = Blog::Post[ params[:post_id] ]
  throw :halt, [404, erb(not_found) ] unless @post
  erb :post 
end