Class Task < ActiveRecord::Base
      belongs_to :sub_task, class_name: Task.name, touch: true
      has_many :sub_tasks, class_name: Task.name, foreign_key: :sub_task_id, dependent: :destroy
    end

class Task < ActiveRecord::Base
   belongs_to :sub_task, class_name: Task.name, touch: true, counter_cache: :sub_tasks_count
   has_many :sub_tasks, class_name: Task.name, foreign_key: :sub_task_id, dependent: :destroy
end