In django it's pretty straightforward, but ruby on rails 3 had me scratching my head for a good long while.
Here's the solution, or at least the parts that were difficult:
<%= link_to "Comments", show_comments_path( :format => 'js', :comment_id=> session[:id] ), :remote => true %>This whole :remote => true business tells rails 3 that this is an ajax call. The :format param does NOT get passed into the url, but DOES render the response as html, most of the time. Which is to say, rails will run the .js.erb file with the name matching your route.
Here's the route:
match 'show_comments/:comment_id', :to => 'deals#show_comments', :as => :show_comments
facepalms: 4 (steady, slow, only slightly infuriating progress)