nested restful route param automagically populated on current page?

I’ve been trying to figure out why this works…

link_to “foo”, admin_comments_url()

comments are a nested route of blogs, so it seems like that method would need the the blog param:


However, when I’m already on this page /admin/blogs/1/comments, I don’t need to pass in the @blog param. Is this something that just happens magically behind the scenes?