old_contents: from django.conf.urls import url
  from . import views
  
  urlpatterns = [
      url(r'^$', views.post_list.as_view()),
      url(r'P<id>[0-9]/$', views.post_detail.as_view())
  ]
new_contents: from django.conf.urls import url
  from . import views
  
  urlpatterns = [
      # ex: /api/threads/
      url(r'^$', views.post_list.as_view()),
      # ex: /api/threads/143/first-post
      url(r'(?P<id>[0-9])/(?P<slug>[-\w\d])/$', views.post_detail.as_view())
  ]
subject: make thread url prefixed with id, then slug