old_contents: from django.contrib import admin
  from forum.models import Category, Forum, Thread
  
  admin.site.register(Category)
  admin.site.register(Forum)
  admin.site.register(Thread)
  
new_contents: from django.contrib import admin
  from forum.models import Category, Forum, Thread
  
  class ForumInline(admin.StackedInline):
      model = Forum
  
  class CategoryAdmin(admin.ModelAdmin):
      inlines = [ForumInline]
  
  admin.site.register(Category, CategoryAdmin)
  admin.site.register(Thread)
  
subject: Modify forums directly in categories.