performance: Avoid Recipient lookup for stream messages.

All the fields of a stream's recipient object can
be inferred from the Stream, so we just make a local
object.  Django will create a Message object without
checking that the child Recipient object has been
saved.  If that behavior changes in some upgrade,
we should see some pretty obvious symptom, including
query counts changing.

Tweaked by tabbott to add a longer explanatory comment, and delete a
useless old comment.

zerver/lib/actions.py

@@ -2335,9 +2335,20 @@ def check_message(sender: UserProfile, client: Client, addressee: Addressee,
             stream = addressee.stream()
         assert stream is not None
 
-        recipient = stream.recipient
+        # To save a database round trip, we construct the Recipient
-
+        # object for the Stream rather than fetching it from the
-        # This will raise JsonableError if there are problems.
+        # database using the stream.recipient foreign key.
+        #
+        # This is simpler than ensuring that code paths that fetch a
+        # Stream that will be used for sending a message have a
+        # `select_related("recipient"), which would also needlessly
+        # expand Stream objects in memory (all the fields of Recipient
+        # are already known given the Stream object).
+        recipient = Recipient(
+            id=stream.recipient_id,
+            type_id=stream.id,
+            type=Recipient.STREAM,
+        )
 
         if sender.bot_type != sender.OUTGOING_WEBHOOK_BOT:
             access_stream_for_send_message(

zerver/tests/test_message_edit.py

@@ -963,7 +963,7 @@ class EditMessageTest(ZulipTestCase):
                 'propagate_mode': 'change_all',
                 'topic': 'new topic',
             })
-        self.assertEqual(len(queries), 51)
+        self.assertEqual(len(queries), 50)
 
         messages = get_topic_messages(user_profile, old_stream, "test")
         self.assertEqual(len(messages), 1)

zerver/tests/test_message_send.py

@@ -1194,7 +1194,7 @@ class StreamMessagesTest(ZulipTestCase):
                 body=content,
             )
 
-        self.assert_length(queries, 14)
+        self.assert_length(queries, 13)
 
     def test_stream_message_dict(self) -> None:
         user_profile = self.example_user('iago')

zerver/tests/test_subs.py

@@ -3838,7 +3838,7 @@ class SubscriptionAPITest(ZulipTestCase):
                     principals=orjson.dumps([user1.id, user2.id]).decode(),
                 ),
             )
-        self.assert_length(queries, 49)
+        self.assert_length(queries, 48)
 
 class GetStreamsTest(ZulipTestCase):
     def test_streams_api_for_bot_owners(self) -> None: