redirect_to_post.rb

class ApplicationController < ActionController::Base

  protected

  def redirect_if_moved(record, param_key = :id)
    redirect_to record, :status => 301 if record.to_param != params[param_key]
  end

end

class PostsController < ApplicationController

  def show
    @post = Post.find(params[:id])
    redirect_if_moved @post
  end

end