Flexibility without Dependency Injection

gistfile1.txt

class TaxCode
  GENERATORS = {
    :us => lambda { |id| "US-#{id}" },
    :br => lambda { |id| "#{id + 9}-BRA" },
  }

  def self.generate(code, id)
    gen = GENERATORS[code] || raise ArgumentError, "No generator for country #{code}"
    gen.call(id)
  end
end

class User

  attr_accessor :country_code
  attr_accessor :id

  def tax_code
    TaxCode.generate(country_code, id)
  end
end