Rails: Nested resources conflict, how to scope the index action depending on the called route

Question

Imagine you have two defined routes:

map.resources articles
map.resources categories, :has_many => :articles

both accessible by helpers/path

Answer 1

if params[:category_id].blank?
  # all
else
  # find by category_id
end

I like to consider the action independent from the route. No matter how they get there, make a reasonable decision as to what to do.