app/models/speach_balloon.rb

   1 class SpeachBalloon < ActiveRecord::Base
   2   has_many :balloon_templates, :dependent => :destroy
   3   accepts_nested_attributes_for :balloon_templates
   4   before_create :supply_t
   5
   6   def supply_t
   7     self.t = SpeachBalloon.maximum(:t).to_i + 1
   8   end
   9
  10 end