old_contents: from django.contrib import admin
  
  from .models import Subscription
  
  
  class SubscriptionAdmin(admin.ModelAdmin):
      list_display = ['__str__', 'is_subscribed']
  
  admin.site.register(Subscription, SubscriptionAdmin)
  
new_contents: from django.contrib import admin
  
  from .models import Subscription
  
  
  class SubscriptionAdmin(admin.ModelAdmin):
      list_display = ['__str__', 'is_subscribed']
      list_filter = ['is_subscribed']
  
  admin.site.register(Subscription, SubscriptionAdmin)
  
subject: Add `is_subscribed` list filter