Rails：如何在具有范围的某些路径上设置参数，而在另一些路径上将其设置为nil？

      resources :cities

      resources :categories

      devise_for :clients
      namespace :clients do
          resources :account
          resources :dashboard
          resources :offers
      end

      scope \"/:current_city\" do 
          scope \"/:current_category\" do
              match \'articles/last_articles\' => \'articles#index_last_articles\', :as => \"last_articles\"
              resources :articles do
                  resources :comments
              end
          end
      end
      root :to => \"home#index\"

:current_city => nil, :current_category => nil