Rails enum getter for transient attributes

transient_enumerable.rb

# Rails enum does not decorate transient attribute getters
# This concern aims to set getters if enum for
# transient attribute has been defined.
# 
# Also enables question? methods.
# 
# Example usage:
# class Model < ActiveRecord::Base
#   include TransientEnumerable
#
#   attr_writer :transient_state # <- please use writer only
#   
#   enum transient_state: {
#     inactive: 0,
#     active: 1
#   }
#
#   transient_enum_decorate :transient_state
#
#   ...
# end
#

module TransientEnumerable
  extend ActiveSupport::Concern

  class_methods do
    def transient_enum_decorate(*attributes)
      attributes.each do |attr|
        mapping = self.send(attr.to_s.pluralize)

        define_method(attr) do
          value = instance_variable_get("@" + attr.to_s)
          return unless value

          if mapping.has_key?(value)
            value.to_s
          elsif mapping.has_value?(value.to_i)
            mapping.key(value.to_i)
          else
            raise ArgumentError, "'#{value}' is not a valid #{attr.to_s}"
          end
        end

        # Redefine enum question? helper methods
        mapping.each do |value, i|
          define_method("#{value}?") { self.public_send(attr) == value.to_s }
        end
      end
    end
  end
end