In Ruby, classes are never closed: you can always add methods to an existing class. This applies to the classes you write as well as the standard, built-in classes. All you have to do is open up a class definition for an existing class, and the new contents you specify will be added to whatever's there. In the same regard, if module is used with the name of a pre-existing module that module is re-opened. If a method is defined in a re-opened module with the same name as a pre-existing method in the same module the old method is overwritten with the new. And, of course, modules can be made immutable, effectively preventing them from being reopened by freezing the module object. Frozen modules raise RuntimeErrors when methods are defined, or variables manipulated, in their context.
When you include a module in a class, without a prefix (prefix meaning something like this: AbstractController::Layouts), then ruby searches for the module of that name within the scope of the current module wrapping the class that the include was called in.
So, for example:
module ActionController class Base include Rendering
Since we "include Rendering" within the scope of the Base class object, ruby first looksup the constant Rendering, that is, looking for a module named Rendering in the ActionController namespace, since that is the namesapce that Base is defined in???
Well, ActionController module does define a module named Rendering:
module ActionController module Rendering def render(*args) raise ::AbstractController::DoubleRenderError if response_body super self.content_type ||= Mime[lookup_context.rendered_format].to_s response_body end
So now the method render(*args) is included in the Base class as an instance method. So when we call, for example, (hypothetically) ActionView::Base.new.render :action => "my_action", :layout => false, ActionView::Base is instantiated and we invoke the render instance method, and since no render method is declared in the Base class context, ruby scope chain does lookup into the module Rendering and finds the method so calls it. Now, as you may know, it's possible to pass multiple kinds of arguments to render, depending on whether you want to render a template or a partial or file, etc. The * syntax does the job here, allowing you vary the number of arguments passed. It checks if there is already a response to the request, and raises an Exception, if so. Otherwise, it calls super.
During this super call, I got a little confused. In ActionController::Base, there is another module included called AbstractController::Layouts. Layouts in turn includes a module called Rendering. Since there is a Rendering module within the AbstractController namespace, it is found, and it indeed contains a method called render.
def render(*args, &block) options = _normalize_render(*args, &block) self.response_body = render_to_body(options) end
And because render is included in Layouts, it's included as an instance method, and because Layouts was included in ActionController::Base, that render method utlimately makes its way up to the Base class. Now we have two modules (ActionController::Rendering and AbstractController::Rendering that are including this method into Base. So why doesn't one of them get overwritten? Why does the call to super work?