Playing around with alternatives to this post: http://kresimirbojcic.com/2011/11/19/dependency-injection-in-ruby.html

use_factory_method.rb

# app/models/customer.rb
class Customer
  attr_reader :id, :country

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

  def tax_code
    TaxCodes.find(country).tax_code(id)
  end
end

# app/models/tax_codes.rb
module TaxCodes
  def self.find(country)
    begin
      TaxCodes.const_get(country)
    rescue NameError
      raise "Tax codes for #{country} not supported"
    end
  end
end

# app/models/tax_codes/united_states.rb
module TaxCodes
  class USA
    def self.tax_code(id)
      "US-#{id}"
    end
  end
end

# app/models/tax_codes/brazil.rb
module TaxCodes
  class Brazil
    def self.tax_code(id)
      "#{id}-BR"
    end
  end
end

puts Customer.new(country: 'USA', id: '1234').tax_code