Rails namespace routes and custom action on resource

Rails

You need to add a collection

namespace :api do
  namespace :v1 do
    resources :users do
      collection do
        get 'all'
      end
    end
    resources :sessions
  end
end
The namespace and resources method calls create the hierarchy; a route nested with the resource :users block will take on the path of it's ancestors. The collection creates nested routes on the resource collection instead of single instances of the resource.

/api/v1/users/all
Recommended Reading: http://guides.rubyonrails.org/routing.html#adding-collection-routes