Challenge 2: Write a post_delete Signal Receiver — Possible Solution
====================================================================

# catalog/signals.py

from django.db.models.signals import post_delete
from django.dispatch import receiver
from .models import Book

@receiver(post_delete, sender=Book)
def log_book_deletion(sender, instance, **kwargs):
    print(f"Book deleted: {instance.title} (id={instance.pk})")

# catalog/apps.py

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 — imported for its side effect (connecting signals)

# mysite/settings.py — make sure the app config is actually used
# (usually already correct by default, but worth confirming):

INSTALLED_APPS = [
    # ...
    "catalog.apps.CatalogConfig",  # or simply "catalog", which Django resolves
                                    # to this same AppConfig by default in modern versions
]

WHY THIS WORKS
--------------
- @receiver(post_delete, sender=Book) connects log_book_deletion
  specifically to Book's post_delete signal — sender=Book means this
  function only fires when a Book instance is deleted, not for every
  model's deletions project-wide.
- post_delete receivers get sender and instance as arguments, but NOT a
  `created` flag (that's specific to post_save) — instance here is the
  object that WAS just deleted, still accessible in memory even though
  its database row is already gone by the time this receiver runs.
- The ready() method on CatalogConfig is the required wiring step: Django
  calls ready() once, when the app registry is fully loaded at startup,
  and importing catalog.signals from inside it is what actually executes
  the @receiver(...) decorator and connects the receiver function to the
  signal. Without this import happening somewhere, signals.py's code
  never runs at all — the receiver function would simply never be
  connected, and Book deletions would proceed silently with no log line,
  with no error anywhere pointing at why.