old_contents: from django.conf.urls import url, patterns
  
  import views
  
  urlpatterns = patterns('',
      url(r'^add', views.CartCreateRest.as_view(), name='add'),
      url(r'^(?P<pk>[0-9]+)$', views.CartDetailView.as_view(), name='detail')
  )
  
new_contents: from django.conf.urls import url, patterns
  
  import views
  
  urlpatterns = patterns('',
      url(r'^add', views.CartCreateRest.as_view(), name='add'),
      url(r'^$', views.CartDetailView.as_view(), name='detail')
  )
  
subject: remove pk arguments from CartDetailView