Rails: Set a common or global instance variable across several controller actions

Firstly, you don't want to try to insert code "between" a controller action and a template rendering. Why? Because you want the controller action to have the freedom to choose what sort of response to give. It could return XML, JSON, headers only, a redirection, nothing, etc. That's why after filters are executed after the response has been rendered.

Secondly, you don't want to monkey patch Fixnum. I mean, maybe you do, but I don't. Not often at least, and not unless I get some totally wicked semantic benefits from it, like being able to say 3.blind_mice. Monkey patching it for a random use case like this seems like a maintenance headache down the road.

You mention refactoring out all the controllers' case specific code into a before filter and running them sequentially. Which brings up to my mind... @foo is the same in every case? If that's the case, then one before filter would work just fine:

before_filter :do_common_stuff
def do_common_stuff
  @foo = common_foo
  @bar = do_something_with @foo
end

That's a totally legit approach. But if @foo changes from controller to controller... well, you have a few more options.

You can separate your before filters into two halves, and customize one per controller.

# application_controller:
before_filter :get_foo, :do_something_common
def do_something_common
  @bar = do_something_with @foo
end

# baz_controller:
def get_foo
  @foo = pull_from_mouth
end

#baf_controller:
def get_foo
  @foo = pull_from_ear
end

But you know, if it's a simple case that doesn't need database access or network access or anything like that... which your case doesn't... don't kill yourself. And don't sweat it. Throw it in a helper. That's what they're there for, to help. You're basically just rearranging some view data into a form slightly easier to use anyway. A helper is my vote. And you can just name it next_url. :)

I would have a method on @foo which returns a bar, that way you can use @foo.bar in your views.

<% @bar = @foo.bar %> #if you really don't want to change your views, but you didn't hear this from me :)

Use <%= do_some_calculations(@foo) %> inside your templates. That is the straight way.