digest: Directly fetch recipient ids from the DB.

Instead of iterating over Subscriptions and creating the list of home view
recipients, the query now only fetches recipient IDs from the DB.

zerver/lib/digest.py

@@ -230,11 +230,10 @@ def handle_digest_email(user_profile_id: int, cutoff: float,
         user_profile, [pm.message for pm in pms[:pms_limit]])
     context['remaining_unread_pms_count'] = max(0, len(pms) - pms_limit)
 
-    home_view_recipients = [sub.recipient for sub in
-                            Subscription.objects.filter(
-                                user_profile=user_profile,
-                                active=True,
-                                in_home_view=True)]
+    home_view_recipients = Subscription.objects.filter(
+        user_profile=user_profile,
+        active=True,
+        in_home_view=True).values_list('recipient_id', flat=True)
 
     stream_messages = all_messages.filter(
         message__recipient__type=Recipient.STREAM,

zerver/tests/test_digest.py

@@ -121,7 +121,7 @@ class TestDigestEmailMessages(ZulipTestCase):
         with queries_captured() as queries:
             handle_digest_email(othello.id, cutoff)
 
-        self.assertTrue(29 <= len(queries) <= 30)
+        self.assertTrue(24 <= len(queries) <= 25)
 
         self.assertEqual(mock_send_future_email.call_count, 1)
         kwargs = mock_send_future_email.call_args[1]