Get scoped object ids without overhead of instantiating full objects

scoped_ids.rb

class Animal < ActiveRecord::Base
  named_scope :with_wings, :conditions => {:wings => true}
  named_scope :with_four_legs, :conditions => {:legs => 4}

  # Returns object ids falling within current scope without
  # instantiating actual model objects
  
  def self.ids
    column = columns_hash[primary_key]
    ids = connection.select_all(construct_finder_sql(:select => primary_key)).collect do |value| 
      v = value[primary_key]
      # TODO Find nicer way to type cast
      eval column.type_cast_code('v')
    end
  end
end

Animal.ids
Animal.with_wings.ids
Animal.with_wings.with_four_legs.ids