Concern that adds _local attr methods to convert times to a given time zone on a per user basis

local_date_time_attr_readers.rb

# ActiveRecord stores timestamps in UTC and converts them into the application's time zone
# (see config.time_zone in config/application.rb).
#
# This module provides a simple to use API to automatically generate '_local' suffix methods
# for model attribute readers that convert dates and times according to a per model time zone.
module LocalDateTimeAttrReaders
  extend ActiveSupport::Concern
  
  included do
    include ActiveModel::AttributeMethods
  end
  
  class_methods do
    # Defines the name of an instance method that returns the time zone
    # to use for time conversions.
    #
    #   class User
    #     include LocalDateTimeAttrReaders
    #
    #     attr_accessor :time_zone
    #
    #     time_zone_attr_reader :time_zone
    #   end
    def time_zone_attr_reader(attr)
      define_method :time_zone_reader do
        self.send(attr)
      end
    end
    
    # Registers attribute reader methods with +_local+ suffix to read
    # +Date+, +Time+ and +DateTime+ typed attributes in local time.
    #
    #   class User
    #     include LocalDateTimeAttrReaders
    #
    #     attr_accessor :time_zone, :birthday
    #
    #     time_zone_attr_reader  :time_zone
    #     local_date_attr_reader :birthday
    #
    #     def initialize(time_zone = "Europe/Berlin", birthday = Date.today)
    #       @time_zone = time_zone
    #       @birthday  = birthday
    #     end
    #   end
    #
    #   user = User.new
    #   user.birthday
    #   => Date instance using global application time zone
    #
    #   user.birthday_local
    #   => Date instance using time zone specified in :time_zone attribute (here: "Europe/Berlin")
    def local_attr_reader(*attrs)
      # We use ActiveModel::AttributeMethods instead of manually registering methods
      attribute_method_suffix '_local'
      
      # Why does this not work?
      # define_attribute_methods *attrs
      attrs.each { |attr| define_attribute_method attr }
    end
    
    alias_method :local_date_attr_reader, :local_attr_reader
    alias_method :local_time_attr_reader, :local_attr_reader
    alias_method :local_datetime_attr_reader, :local_attr_reader
  end
  
  private
  
    def attribute_local(attr)
      send(attr).in_time_zone(time_zone_reader)
    end
end