gistfile1.rb

  define_builder(Country) do |klass, overrides|
    # set up some presets
    canada = { :iso => 'CA', :name => 'CANADA', :printable_name => 'Canada', :iso3 => 'CAN', :numcode => '124'}
    usa = { :iso => 'US', :name => 'UNITED STATES', :printable_name => 'United States', :iso3 => 'USA', :numcode => '840' }
    uk = { :iso => 'GB', :name => 'UNITED KINGDOM', :printable_name => 'United Kingdom', :iso3 => 'GBR', :numcode => '826' }
    zimbabwe = { :iso => 'ZW', :name => 'ZIMBABWE', :printable_name => 'Zimbabwe', :iso3 => 'ZWE', :numcode => '716' }
    # set up the default
    country_attributes = canada
    # check for a preset
    overrides.process(:preset) do |preset|
      case preset.downcase
        when "usa"
          overrides = usa
        when "uk"
          overrides = uk
        when "other"
          overrides = zimbabwe
      end
    end
    klass.new(country_attributes)
  end

--
>> new_country(:preset => "uk")
=> #<Country id: nil, iso: "CA", name: "CANADA", printable_name: "Canada", iso3: "CAN", numcode: 124>