topics history: Simplify data in /topics endpoint.

In anticipation of have all unread message ids available to the
web app in page_params (via a separate effort), we are simplifying
the /topics endpoint to no longer return unread counts.

Instead we have a list of tiny dictionaries with these fields:

    name - name of the topic
    max_id - max message id for the topic (aka most recent)

The items in the list are order by most-recent-topic-first.

zerver/lib/actions.py

@@ -174,49 +174,42 @@ def realm_user_count(realm):
     return UserProfile.objects.filter(realm=realm, is_active=True, is_bot=False).count()
 
 def get_topic_history_for_stream(user_profile, recipient):
-    # type: (UserProfile, Recipient) -> List[Tuple[str, int]]
+    # type: (UserProfile, Recipient) -> List[Dict[str, Any]]
 
-    # We tested the below query on some large prod datasets, and we never
-    # saw more than 50ms to execute it, so we think that's acceptable,
-    # but we will monitor it, and we may later optimize it further.
     query = '''
-        SELECT topic, read, count(*)
-        FROM (
-            SELECT
-                ("zerver_usermessage"."flags" & 1) as read,
-                "zerver_message"."subject" as topic,
-                "zerver_message"."id" as message_id
-            FROM "zerver_usermessage"
-            INNER JOIN "zerver_message" ON (
-                "zerver_usermessage"."message_id" = "zerver_message"."id"
-            ) WHERE (
-                "zerver_usermessage"."user_profile_id" = %s AND
-                "zerver_message"."recipient_id" = %s
-            ) ORDER BY "zerver_usermessage"."message_id" DESC
-        ) messages_for_stream
-        GROUP BY topic, read
-        ORDER BY max(message_id) desc
+        SELECT
+            "zerver_message"."subject" as topic,
+            max("zerver_message".id) as max_message_id
+        FROM "zerver_message"
+        INNER JOIN "zerver_usermessage" ON (
+            "zerver_usermessage"."message_id" = "zerver_message"."id"
+        )
+        WHERE (
+            "zerver_usermessage"."user_profile_id" = %s AND
+            "zerver_message"."recipient_id" = %s
+        )
+        GROUP BY (
+            "zerver_message"."subject"
+        )
+        ORDER BY max("zerver_message".id) DESC
     '''
     cursor = connection.cursor()
     cursor.execute(query, [user_profile.id, recipient.id])
     rows = cursor.fetchall()
     cursor.close()
 
-    topic_names = dict()  # type: Dict[str, str]
-    topic_counts = dict()  # type: Dict[str, int]
-    topics = []
-    for row in rows:
-        topic_name, read, count = row
-        if topic_name.lower() not in topic_names:
-            topic_names[topic_name.lower()] = topic_name
-        topic_name = topic_names[topic_name.lower()]
-        if topic_name not in topic_counts:
-            topic_counts[topic_name] = 0
-            topics.append(topic_name)
-        if not read:
-            topic_counts[topic_name] += count
+    canonical_topic_names = set()  # type: Set[str]
+    history = []
+    for (topic_name, max_message_id) in rows:
+        canonical_name = topic_name.lower()
+        if canonical_name in canonical_topic_names:
+            continue
+
+        canonical_topic_names.add(canonical_name)
+        history.append(dict(
+            name=topic_name,
+            max_id=max_message_id))
 
-    history = [(topic, topic_counts[topic]) for topic in topics]
     return history
 
 def send_signup_message(sender, signups_stream, user_profile,

zerver/tests/test_messages.py

@@ -70,8 +70,8 @@ class TopicHistoryTest(ZulipTestCase):
         stream = get_stream(stream_name, user_profile.realm)
         recipient = get_recipient(Recipient.STREAM, stream.id)
 
-        def create_test_message(topic, read, starred=False):
-            # type: (str, bool, bool) -> None
+        def create_test_message(topic):
+            # type: (str) -> int
 
             hamlet = self.example_user('hamlet')
             message = Message.objects.create(
@@ -82,28 +82,31 @@ class TopicHistoryTest(ZulipTestCase):
                 pub_date=timezone_now(),
                 sending_client=get_client('whatever'),
             )
-            flags = 0
-            if read:
-                flags |= UserMessage.flags.read
-
-            # use this to make sure our query isn't confused
-            # by other flags
-            if starred:
-                flags |= UserMessage.flags.starred
 
             UserMessage.objects.create(
                 user_profile=user_profile,
                 message=message,
-                flags=flags,
+                flags=0,
             )
 
-        create_test_message('topic2', read=False)
-        create_test_message('toPIc1', read=False, starred=True)
-        create_test_message('topic2', read=False)
-        create_test_message('topic2', read=True)
-        create_test_message('topic2', read=False, starred=True)
-        create_test_message('Topic2', read=False)
-        create_test_message('already_read', read=True)
+            return message.id
+
+        # our most recent topics are topic0, topic1, topic2
+
+        # Create old messages with strange spellings.
+        create_test_message('topic2')
+        create_test_message('toPIc1')
+        create_test_message('toPIc0')
+        create_test_message('topic2')
+        create_test_message('topic2')
+        create_test_message('Topic2')
+
+        # Create new messages
+        topic2_msg_id = create_test_message('topic2')
+        create_test_message('topic1')
+        create_test_message('topic1')
+        topic1_msg_id = create_test_message('topic1')
+        topic0_msg_id = create_test_message('topic0')
 
         endpoint = '/json/users/me/%d/topics' % (stream.id,)
         result = self.client_get(endpoint, dict())
@@ -112,10 +115,18 @@ class TopicHistoryTest(ZulipTestCase):
 
         # We only look at the most recent three topics, because
         # the prior fixture data may be unreliable.
-        self.assertEqual(history[:3], [
-            [u'already_read', 0],
-            [u'Topic2', 4],
-            [u'toPIc1', 1],
+        history = history[:3]
+
+        self.assertEqual([topic['name'] for topic in history], [
+            'topic0',
+            'topic1',
+            'topic2',
+        ])
+
+        self.assertEqual([topic['max_id'] for topic in history], [
+            topic0_msg_id,
+            topic1_msg_id,
+            topic2_msg_id,
         ])
 
     def test_bad_stream_id(self):

zerver/views/streams.py

@@ -377,9 +377,6 @@ def get_topics_backend(request, user_profile,
         recipient=recipient,
     )
 
-    # Our data structure here is a list of tuples of
-    # (topic name, unread count), and it's reverse chronological,
-    # so the most recent topic is the first element of the list.
     return json_success(dict(topics=result))
 
 @authenticated_json_post_view