Challenge 3: Invalidate the Cache on Save — Possible Solution
====================================================================

# catalog/signals.py

from django.core.cache import cache
from django.db.models.signals import post_save, post_delete
from django.dispatch import receiver
from .models import Book

@receiver(post_save, sender=Book)
def invalidate_book_stats_on_save(sender, instance, **kwargs):
    cache.delete("book_stats")

@receiver(post_delete, sender=Book)
def invalidate_book_stats_on_delete(sender, instance, **kwargs):
    cache.delete("book_stats")

# catalog/apps.py — required wiring, per Chapter 4's gotcha

from django.apps import AppConfig

class CatalogConfig(AppConfig):
    default_auto_field = "django.db.models.BigAutoField"
    name = "catalog"

    def ready(self):
        import catalog.signals  # noqa: F401

WHY THIS WORKS
--------------
- Two separate receivers cover both ways the cached stats could go
  stale: post_save fires for both NEW books being created and EXISTING
  books being updated (e.g. a price correction), while post_delete
  handles a book being removed entirely — both cases change what
  get_catalog_stats() from Challenge 1 would compute.
- cache.delete("book_stats") removes the cached entry immediately —
  the NEXT call to cache.get_or_set("book_stats", get_catalog_stats, ...)
  after a save or delete will find nothing cached, so it recomputes the
  aggregate query fresh and caches the new, correct result. This is
  strictly better than waiting out the full 10-minute timeout from
  Challenge 1, during which the displayed stats would otherwise be
  visibly wrong.
- This directly reuses the signal + apps.py wiring pattern from
  Chapter 4 — the same "sender doesn't need to know who's listening"
  decoupling applies here: nothing in the view or admin code that saves a
  Book needs to remember to also clear the cache; the signal handles it
  automatically, everywhere a Book is saved or deleted throughout the
  project (including the admin site from Course 1's final chapter).
- As with any signal, the apps.py ready() import is what actually
  connects these receivers — without it, book saves would proceed
  silently with a now-stale cache never being cleared, with no error
  indicating why the stats look wrong.