old_contents: from django.views.generic import ListView
  
  from models import Category
  
  
  class CategoryListView(ListView):
      model = Category
  
new_contents: from django.views.generic import ListView, DetailView
  
  from models import Category, Forum
  
  
  class CategoryListView(ListView):
      model = Category
  
  class ForumDetailView(DetailView):
      model = Forum
  
subject: implement forum detail view