Sluggable : An ActiveSupport Concern for generating URL slugs on a Model.

post.rb

# === Table Schema
#
#  id:        integer
#  title:     string
#  body:      string
#  author_id: integer
#
class Post < ActiveRecord::Base
  include Sluggable
  belongs_to :author
  slugify :title, scope: :author
end

# Slug is automagically generated before Validation:
Post.new(title: 'Something Awesome')
Post.slug   #=> nil
Post.valid? #=> true
Post.slug   #=> 'something-awesome'
Post.save

# Slug is generated unique in scope (here, scope is nil author):
Post.create(title: 'Something Awesome')
Post.slug   #=> 'something-awesome-1'

# Slug can be manually set and this concern won't do much of anything:
Post.create(title: 'Something Awesome', slug: 'rick-astley')

sluggable.rb

module Sluggable
  extend ActiveSupport::Concern

  included do
    before_validation :generate_slug
    class_attribute :slug_options
    class_attribute :slug_source

    self.slug_source  = :name
    self.slug_options = {}
  end

  def to_param
    self.slug
  end

  module ClassMethods
    def slugify(source_column, options = {})
      self.slug_source  = source_column
      self.slug_options = options
    end
  end

  private

  def generate_slug
    return true unless self.slug.blank?

    slug_scope = slug_options[:scope] ? {slug_options[:scope] => self.send(slug_options[:scope])} : '1=1'

    count = 0

    begin
      self.slug = to_slug(self.send(slug_source), count > 0 ? count : nil)
      count += 1
    end while self.class.where(slug_scope).exists?(slug: self.slug)

    true
  end

  def to_slug(string, tail=nil)
    return if string.nil?
    slug = string.downcase.gsub(/[^a-z0-9-]+/, '-').gsub(/^-+|-+$/, '')
    slug = [slug, tail].join('-') if tail
    slug
  end
end