gistfile1.rbx

module EnumThing
  def categories
    @categories ||= connection.execute("SELECT category FROM #{table_name} GROUP BY category").collect{|r|r[0]}
  end
  include InstanceMethods
  
  module InstanceMethod
    def categories
      self.class.categories
    end
  end
end

class Book < ActiveRecord::Base
  extend EnumThing
end