Examples of advanced Rails 3.0 routes

routes.rb

Rails.application.routes.draw do

  get '/(:locale)/products/(:category)/(page/:page).:extension',
        :to => 'products#index',
        :as => :products,
        :constraints => {
          :locale => /[a-z]{2}/,
          :category => /.+?/,
          :page => /\d+/
        },
        :defaults => {
          :page => 1,
          :extension => 'html',
          :locale => 'en'
        }

  # products_path(:page => 1)
  # => /products.html

  # products_path(:page => 2)
  # => /products/page/2.html

  # products_path(:page => 2, :locale => 'de')
  # => /de/products/page/2.html

  # products_path('computers', :page => 2, :locale => 'de')
  # => /de/products/computers/page/2.html

  # products_path('computers/apple', :page => 2, :locale => 'de')
  # => /de/products/computers/apple/page/2.html

  # products_path('computers/apple')
  # => /products/computers/apple.html

end

So when you write:
products_path('computers', :page => 2, :locale => 'de')
How does the method know that computers is the category when its not being specified like the other params?