old_contents: from django.conf.urls import url
  from product.views import ProductCategoryList, ProductCategoryDetail, ProductCategoryCreate
  
  urlpatterns = [
      url(r'^category/', ProductCategoryList.as_view()),
      url(r'^category/detail/(?P<pk>[0-9]*)/$', 
          ProductCategoryDetail.as_view(),
          name='product-category-detail'),
      url(r'^category/add$', 
          ProductCategoryCreate.as_view(), 
          name='product-category-add')
  ]
  
new_contents: from django.conf.urls import url
  from product.views import ProductCategoryList, ProductCategoryDetail, ProductCategoryCreate
  
  urlpatterns = [
      url(r'^category/$', ProductCategoryList.as_view()),
      url(r'^category/detail/(?P<pk>[0-9]*)/$', 
          ProductCategoryDetail.as_view(),
          name='product-category-detail'),
      url(r'^category/add$', 
          ProductCategoryCreate.as_view(), 
          name='product-category-add')
  ]
  
subject: Fix URL.