another variation of constant loading with metaprogramming

country_storage.rb

module CountryStorage

  private

  # extend this module than call some constant
  # using agreement COUNTRY_#{country_code}
  # this will raise const_missing and than create constant COUNTRY_#{country_code}
  # equal to Country.find_by(code: country_code)

  # instead of using
  # COUNTRY_ID = Country.find_by_code('US').id.freeze
  # which may fall down your app if you trying to load unit when you don't have countries in DB

  def const_missing(const_name)
    country = nil
    if const_name =~ /COUNTRY_(\S+)/
      code = const_name.to_s.match(/COUNTRY_(\S+)/)[1]
      country = set_country_const(const_name, code)
    end
    country || super
  end

  def set_country_const(name, code)
    country = get_country(code)
    if country
      const_set(name , country)
    end
    return country
  end

  def get_country(code)
    Country.find_by(code: code)
  end

end

# class CountryStorageUser
#   extend CountryStorage
#
#   def initialize()
#     us = COUNTRY_US
#     ru = COUNTRY_RU
#     us = COUNTRY_US
#   end
#
# end

# === USAGE
# [1] pry(main)> CountryStorageUser.new
# Country Load (0.5ms)  SELECT  "countries".* FROM "countries"  WHERE "countries"."code" = 'US' LIMIT 1
# Country Load (0.3ms)  SELECT  "countries".* FROM "countries"  WHERE "countries"."code" = 'RU' LIMIT 1