gistfile1.rb

# app/controllers/posts_controller.rb
class PostsController < ApplicationController
  
  before_filter :find_post
  
  protected
    
    def find_post
      unless @post = Post.where(:slug => params[:id]).first
        # what do we do here?
      end
    end
end

# app/models/post.rb
class Post < ActiveRecord::Base
  def to_param
    self.slug
  end
end

def find_post
  unless @post = Post.find_by(slug: params[:id])
    # what do we do here?
  end
end

More understandable :)