default_values.rb

module DefaultValues

  def has_default_values(default_values = {})

    class_attribute :default_values
    self.default_values = default_values

    after_initialize :assign_default_values

    include InstanceMethods
  end

  module InstanceMethods
    private

    def assign_default_values
      return unless new_record?
      default_values.each do |key, value|
        self[key] = value if self[key].nil?
      end
    end
  end

end

extend_active_record.rb

# In config/initializers
ActiveRecord::Base.extend DefaultValues

some_model.rb

# Example usage
class SomeModel < ActiveRecord::Base

  has_default_values(
    title: "Default",
    flag_yn: false
  )
  
  ..

end