Prevent book being deleted until all its instances are deleted (mdn#86)

* Cascade book deletion

* Restrict deletion of book

Author: hamishwillee

catalog/models.py

@@ -71,7 +71,7 @@ class BookInstance(models.Model):
     """Model representing a specific copy of a book (i.e. that can be borrowed from the library)."""
     id = models.UUIDField(primary_key=True, default=uuid.uuid4,
                           help_text="Unique ID for this particular book across whole library")
-    book = models.ForeignKey('Book', on_delete=models.SET_NULL, null=True)
+    book = models.ForeignKey('Book', on_delete=models.RESTRICT)
     imprint = models.CharField(max_length=200)
     due_back = models.DateField(null=True, blank=True)
     borrower = models.ForeignKey(User, on_delete=models.SET_NULL, null=True, blank=True)