code: def get_urls(self):
          
          base_urls = super(SimpleRouter, self).get_urls()
          single_urls = sum([r.urls for r in self._single_object_registry], [])
          nested_urls = sum([r.urls for r in self._nested_object_registry], [])
  
          return base_urls + single_urls + nested_urls
docs: Generate the list of URL patterns including the registered single
          object routers urls.