add_touch_callbacks

# File activerecord/lib/active_record/associations/builder/belongs_to.rb, line 105
    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, belongs_to_touch_method)
      }

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

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

    def self.define_validations(model, reflection)
      if reflection.options.key?(:required)
        reflection.options[:optional] = !reflection.options.delete(:required)
      end

      if reflection.options[:optional].nil?
        required = model.belongs_to_required_by_default
      else
        required = !reflection.options[:optional]
      end

      super

      if required
        model.validates_presence_of reflection.name, message: :required
      end
    end
  end