Linkable concern instead of friendly_id gem

linkable.rb

# app/models/concerns/linkable.rb
#
# Generate link that you can use to find resources
# Similar to friendly_id gem but much simpler (one file) and you can easily
# update instead of reading DSL for friendly_id
# You just need to define where to store link and which fields to use
# to generate link
#
# For example:
# rails g migration add_link_to_users link:string
# puts also:     add_index :users, :link, unique: true
# rake db:migrate
# You can generate links for exists users
# User.all.map { |u| u.generate_link; u.save! }
#
#    class User < ActiveRecord::Base
#      include Linkable
#      linkable :link, # where to store generated url
#        [:business_name, :city], # try first with business name and city
#        [:first_name, :last_name, :city], # if business_name is nil, use first_name, last_name
#        [:business_name], # fall back if city is nil
#        [:first_name, :last_name] # fallback if business_name is nil
#     end
#
#  You can find users by link
#  User.find_by link: link
#  you can change resources param name:
#  resources :users, param: :link
#  but it's better to create another route since link could change, but id won't
#  resources :users do member do get 'by-link' end end
module Linkable
  extend ActiveSupport::Concern

  included do
    def self.linkable(link_field, *candidate_field)
      @@linkable_link_field = link_field
      @@linkable_candidate_field = candidate_field
      before_validation :generate_link, if: ->(obj) do
        # TODO: just in case is changed, for example address_changed?
        true
      end
    end
  end

  def generate_link
    candidate_text = find_candidate_text
    if candidate_text.blank?
      errors.add(:base, "Can't generate link")
      return
    end
    link = prepare_link candidate_text
    link = check_duplication link
    send "#{@@linkable_link_field}=", link
  end

  private

  def find_candidate_text
    candidate_text = nil
    if @@linkable_candidate_field.class == Array
      @@linkable_candidate_field.each do |candidate_field|
        if candidate_field.class == Array
          candidate_text_array = candidate_field.map { |f| send f }
          next if candidate_text_array.select(&:blank?).present?
          candidate_text = candidate_text_array.join('-')
        else
          candidate_text = send candidate_field
        end
        break if candidate_text.present?
      end
    else
      candidate_text = send @@linkable_candidate_field
    end
    candidate_text
  end

  def check_duplication(link)
    new_link = link
    num = 1
    while self.class.where(@@linkable_link_field => new_link).where.not(id: id).exists?
      num += 1
      new_link = "#{link}-#{num}"
    end
    new_link
  end

  def prepare_string(input)
    # http://stackoverflow.com/questions/20224915/iconv-will-be-deprecated-in-the-future-transliterate
    result_ascii = ActiveSupport::Multibyte::Unicode
                   .normalize(input, :kd)
                   .chars.grep(/\p{^Mn}/)
                   .join('')

    result = result_ascii.strip.downcase.gsub(/\s+/, ".")
    result
  end

  def prepare_link(input)
    temp = prepare_string input
    temp = temp.tr(".", "-") # dot . conflicts with rails routes request type
    temp = temp.tr(",", "-") # comma , conflicts with joining customers website maps with ,
    temp = temp.gsub(/-+/, '-') # replace one or more --- with -
    temp
  end
end