old_contents: from rest_framework import routers
  from .views import (UserViewSet, SubjectViewSet,
                      SubjectDetailViewSet, SessionDetailViewSet)
  
  router = routers.SimpleRouter()
  router.register(r'user', UserViewSet)
  router.register(r'subjects', SubjectViewSet)
  router.register(r'subject_detail', SubjectDetailViewSet)
  router.register(r'session_detail', SessionDetailViewSet)
  
new_contents: from rest_framework import routers
  from .views import (UserViewSet, SubjectViewSet,
                      SubjectDetailViewSet, SessionDetailViewSet)
  
  router = routers.SimpleRouter()
  router.register(r'user', UserViewSet)
  router.register(r'subjects', SubjectViewSet)
  router.register(r'subject[_-]detail', SubjectDetailViewSet)
  router.register(r'session[_-]detail', SessionDetailViewSet)
  
subject: Allow underscore or dash in URL.