a.rb

    root :to => 'communities#index', :as => :root

    scoped_community_routes = proc do
      resources :topics do
      end
      resources :replies do
      end
      resources :profiles do
      end
    end

    instance_eval(&scoped_community_routes)

    resources :communities do
    end

    scope ':community' do
      match '' => 'communities#show'
      match '/' => 'communities#show'
      instance_eval(&scoped_community_routes)
    end