gistfile1.rb

# app/models/post.rb
class Post < ActiveRecord::Base
  
  validate :ensure_slug_uniqueness
  
  protected
  
    # validate
    def ensure_slug_uniqueness
      
      # we also want to ensure the slug is not blank
      if self.slug.blank?
        errors.add(:slug, "can't be blank")
      end
      
      # if this is a new post, the id is nil
      # otherwise, the slug should point to this post's id
      unless Slug[self.slug].nil? || Slug[self.slug] == self.id.to_s
        errors.add(:slug, "is already taken")
      end
    end
end