add_touch_callbacks

# File activerecord/lib/active_record/associations/builder/belongs_to.rb, line 78
    def self.add_touch_callbacks(model, reflection)
      foreign_key = reflection.foreign_key
      name        = reflection.name
      touch       = reflection.options[:touch]

      callback = lambda { |changes_method| lambda { |record|
        BelongsTo.touch_record(record, record.send(changes_method), foreign_key, name, touch, belongs_to_touch_method)
      }}

      if reflection.counter_cache_column
        touch_callback = callback.(:saved_changes)
        update_callback = lambda { |record|
          instance_exec(record, &touch_callback) unless association(reflection.name).target_changed?
        }
        model.after_update update_callback, if: :saved_changes?
      else
        model.after_create callback.(:saved_changes), if: :saved_changes?
        model.after_update callback.(:saved_changes), if: :saved_changes?
        model.after_destroy callback.(:changes_to_save)
      end

      model.after_touch callback.(:changes_to_save)
    end

    def self.add_default_callbacks(model, reflection)
      model.before_validation lambda { |o|
        o.association(reflection.name).default(&reflection.options[:default])
      }
    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

    private_class_method :macro, :valid_options, :valid_dependent_options, :define_callbacks, :define_validations,
      :add_counter_cache_callbacks, :add_touch_callbacks, :add_default_callbacks, :add_destroy_callbacks
  end
end