use_factory_method.rb

class Customer
  def initialize(attrs = {})
    @id = attrs[:id]
    @country = attrs[:country]
  end

  def tax_code
    TaxCodes[@country].call(@id)
  rescue
    raise "Tax codes for #{@country} not supported"
  end
end

TaxCodes = {
  'Brazil' => lambda { |id| "#{id}-BR" },
  'USA' => lambda { |id| "US-#{id}" }
}