default_values.rb

#
# 
# Rails 4 concern for specifying default attribute values on models
#
#  E.g. The following user has defaults defined for `active` and `manager` attributes 
#
#   class User < ActiveRecord::Base
#     include Concerns::DefaultValues
#
#     default_value :active, true
#
#     default_value :manager do 
#       User.first
#     end
#
#   end
#
# 
#  [email protected] 
#
module Concerns::DefaultValues
  extend ActiveSupport::Concern

  module ClassMethods

    def default_value(attribute, default = nil, &block)
      default = block if block_given?
      self.class_eval do

        before_validation(on: :create) do
          default_value = default.respond_to?(:call) ? default.call : default
          write_attribute(attribute, default_value) unless read_attribute(name)
        end

      end

    end

  end

end