V2: Fixing stale/nonexisting objects.

bookshop/models.py

@@ -2,18 +2,20 @@
 from django.core.cache import cache
 import time
 
+
 class Author(models.Model):
     name = models.CharField(max_length=255)
 
     def get_top_books(self):
         cache_key = 'Author:get_top_books:{}'.format(self.id)
         cache_value = cache.get(cache_key)
         if cache_value is not None:
-            return cache_value
+            return list(Book.objects.filter(id__in=cache_value))
 
         time.sleep(5)  # make the calculation slower for the sake of argument
         books = list(Book.objects.order_by('-purchase_count')[:10])
-        cache.set(cache_key, books, 4 * 60 * 60)  # cache for 4 hours
+        # cache.set(cache_key, books, 4 * 60 * 60)  # cache for 4 hours
+        cache.set(cache_key, [book.id for book in books], 4 * 60 * 60)  # cache for 4 hours
         return books
 
     def __str__(self):