Extracting to_param, find_one functionality out of friendly_id gem

slug.rb

# *****************************************
# *                                       *
# * extracted from                        *
# * https://github.com/norman/friendly_id *
# * Version: 4.0.8                        *
# *                                       *
# * modified by                           *
# * Benjamin Knofe (github.com/benben)    *
# *                                       *
# *****************************************
#
# Example Usage:
#
# class Post < ActiveRecord::Base
#   extend  MyApp::Slug
# end
#
# License:
# 
# Copyright (c) 2008-2012 Norman Clarke and contributors, released under the MIT license.
#
# Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:
# The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

module MyApp
  module Slug

    def self.extended(model_class)
      class << model_class
        alias relation_without_slug relation
      end
      model_class.instance_eval do
        extend Base
        include Model
      end
      @relation_class = model_class.send(:relation_class)
    end

    module Base

      private

      def relation
        @relation = nil unless @relation.class <= relation_class
        @relation ||= relation_class.new(self, arel_table)
        super
      end

      def relation_class
        @relation_class or begin
          @relation_class = Class.new(relation_without_slug.class) do
            alias_method :find_one_without_slug, :find_one
            alias_method :exists_without_slug?, :exists?

            protected

            def is_sluggy? id
              unsluggy_classes = [ActiveRecord::Base, Array, Hash, NilClass, Numeric,
                              Symbol, TrueClass, FalseClass]

              if unsluggy_classes.detect {|klass| id.class <= klass}
                false
              elsif id.respond_to?(:to_i) && id.to_i.to_s != id.to_s
                true
              end
            end

            def find_one id
              return super unless is_sluggy? id
              where(slug: id).first or super
            end

            def exists? id = false
              return super unless is_sluggy? id
              super 'slug' => id
            end
          end
        end
      end
    end

    module Model
      def to_param
        if diff = changes[:slug]
          diff.first || diff.second
        else
          slug.presence || super
        end
      end
    end
  end
end