generate_slug.rb

generate slug

class Post < ActiveRecord::Base
  before_validation     :generate_slug

  def generate_slug
    self.slug = self.title.dup if self.slug.blank?
    self.slug.slugorize!
  end
end

and in lib/core_extensions/string.rb

module CoreExtensions::String
  def slugorize
    result = self.downcase
    result.gsub!(/&(\d)+;/, '')  # Ditch Entities
    result.gsub!('&', 'and')     # Replace & with 'and'
    result.gsub!(/['"]/, '')    # replace quotes by nothing
    result.gsub!(/\W/, ' ')     # strip all non word chars
    result.gsub!(/\ +/, '-')    # replace all white space sections with a dash
    result.gsub!(/(-)$/, '')    # trim dashes
    result.gsub!(/^(-)/, '')    # trim dashes
    result.gsub!(/[^a-zA-Z0-9\-]/, '-') # Get rid of anything we don't like
    result
  end

  def slugorize!
    self.replace(self.slugorize)
  end
end

String.send(:include, CoreExtensions::String)

create initializer/ext.rb

require 'core_extensions/string'