In Ruby or Rails, why is "include" sometimes inside the class and sometimes outside the class?

I thought

class ApplicationController < ActionController::Base
  include Foo

is to add a "mixin" -- so that all methods in the Foo module are treated as methods of the ApplicationController.

But now I see code that is

include Bar

class Applicati开发者_开发知识库onController < ActionController::Base
  include Foo

So why is it outside of ApplicationController? How is that different from the more common use of putting it inside of ApplicationController?

Yes, include Foo inside a class adds Foo to that class's ancestors and thus makes all of Foo's instance methods available to instances of those class.

Outside of any class definition include Foo will add Foo to the ancestors of Object. I.e. it is the same as if you did include Foo inside the definition of the Object class. The use doing this is that all of Foo's instance methods are now available everywhere.