ar_playground_for_so.rb

#!/usr/bin/env ruby
%w|rubygems active_record irb|.each {|lib| require lib}
ActiveSupport::Inflector.inflections.singular("toyota", "toyota")

ActiveRecord::Base.logger = Logger.new(STDOUT)
ActiveRecord::Base.establish_connection(
  :adapter => "sqlite3",
  :database => ":memory:"
)

ActiveRecord::Schema.define do
  create_table :cars do |t|
    t.string :name
  end

  create_table :car_to_wheel_table_map, :id => false do |t|
    t.integer :car_id
    t.string :wheel_table
  end

  create_table :wheels_for_fords do |t|
    t.integer :car_id
    t.string :color
  end

  create_table :wheels_for_toyotas do |t|
    t.integer :car_id
    t.string :color
  end
end

class Wheel < ActiveRecord::Base
  set_table_name nil
  belongs_to :car
end

class CarWheelMap < ActiveRecord::Base
  set_table_name "car_to_wheel_table_map"
  belongs_to :car
end

class Car < ActiveRecord::Base
  has_one :car_wheel_map
  delegate :wheel_table, :to => :car_wheel_map

  def wheels
    @wheels ||= begin
      the_klass = "#{wheel_table.classify}Wheel"
      eval <<-END
        class #{the_klass} < ActiveRecord::Base
          set_table_name "wheels_for_#{wheel_table.pluralize}"
          belongs_to :car
        end
      END
      
      self.class.send(:has_many, "#{wheel_table}_wheels")
      send "#{wheel_table}_wheels"
    end
  end
end

rav4 = Car.create(:name => "Rav4")
rav4.create_car_wheel_map(:wheel_table => "toyota")

fiesta = Car.create(:name => "Fiesta")
fiesta.create_car_wheel_map(:wheel_table => "ford")

rav4.wheels.create(:color => "red")
fiesta.wheels.create(:color => "green")

# IRB.start if __FILE__ == $0