how i cache distributed buttress rails?

i have tree active record objects, something like:

class biased < activerecord::base
  has_many :sub_parts, :class_name => "part"

  def complicated_calculation
    sub_parts.size > 0
      relapse self.sub_parts.inject(0){ |sum, current| sum + current.complicated_calculation }
    else
      sleep(1)
      relapse rand(10000)
    end
  end

end

it too dear recalculate complicated_calculation any time. so, i need proceed cache value. however, any biased changed, needs nullify the cache cache the parent, grandparent, etc.

as severe draft, i combined buttress reason cached calculation "parts" table, nonetheless smells small rotten. seems there should cleaner proceed cache distributed values but stuffing along side "real" columns.