add_touch_callbacks

# File activerecord/lib/active_record/associations/builder/belongs_to.rb, line 97
    def self.add_touch_callbacks(model, reflection)
      foreign_key = reflection.foreign_key
      n           = reflection.name
      touch       = reflection.options[:touch]

      callback = lambda { |record|
        BelongsTo.touch_record(record, foreign_key, n, touch)
      }

      model.after_save    callback, if: :changed?
      model.after_touch   callback
      model.after_destroy callback
    end

    def self.add_destroy_callbacks(model, reflection)
      name = reflection.name
      model.after_destroy lambda { |o| o.association(name).handle_dependency }
    end
  end