tokenable.rb

#
# Generate a unique token before the creating a
# new record. Option to specify an array of
# attributes as predicates for the uniqueness check.
# The tokenable attribute is always appended to
# the list of predicates.
#
# Example usage in a model:
#
#   class Contribution < ActiveRecord::Base
#     include Tokenable
#     # The 'by: []' argument is optional
#     tokenable_attr :payment_token, by: [:contributor_id, :project_id]
#
#     # ...
#   end
#
module Tokenable
  extend ActiveSupport::Concern

  included do |klass|
    def klass.tokenable_attr(name, options = {})
      before_create do |k|
        k.generate_token name, options[:by]
      end
    end

    protected

    def generate_token(attribute, predicates = [])
      predicates = Hash[predicates.map { |n| [n, self[n]] }]
      self[attribute] = loop do
        random_token = SecureRandom.urlsafe_base64(nil, false)
        break random_token unless self.class.exists?(predicates.merge(attribute => random_token))
      end
    end
  end
end