slugable.rb

# app/models/concerns/slugable.rb

module Slugable
  extend ActiveSupport::Concern

  included do
    validates_format_of :slug, :without => /^\d/
  end

  module ClassMethods
    def find input
      input.to_i == 0 ? find_by_slug(input) : super
    end
  end

  def generate_slug
    self.slug = title.parameterize
  end

  def to_param
    slug
  end
  
  def save
    generate_slug
    super
  end
end