Avoid server error when encountering broken edition

If an edition is missing its work, this change allows the page to still
load without a server error; this is important because otherwise the
book will break every page it appears on, including the feed page.

bookwyrm/templatetags/rating_tags.py

@@ -12,6 +12,10 @@ register = template.Library()
 @register.filter(name="rating")
 def get_rating(book, user):
     """get the overall rating of a book"""
+    # this shouldn't happen, but it CAN
+    if not book.parent_work:
+        return None
+
     return cache.get_or_set(
         f"book-rating-{book.parent_work.id}",
         lambda u, b: models.Review.objects.filter(

bookwyrm/tests/templatetags/test_rating_tags.py

@@ -71,6 +71,12 @@ class RatingTags(TestCase):
         )
         self.assertEqual(rating_tags.get_rating(self.book, self.local_user), 5)
 
+    def test_get_rating_broken_edition(self, *_):
+        """Don't have a server error if an edition is missing a work"""
+        broken_book = models.Edition.objects.create(title="Test")
+        broken_book.parent_work = None
+        self.assertIsNone(rating_tags.get_rating(broken_book, self.local_user))
+
     def test_get_user_rating(self, *_):
         """get a user's most recent rating of a book"""
         with patch("bookwyrm.models.activitypub_mixin.broadcast_task.apply_async"):