Track latest topic for each schedule

This adds a new latest_topic_id column to topic_schedule and uses
triggers on the topics table to keep it correct.

This isn't really ideal, but it will simplify a few things related to
scheduled topics by quite a bit. For example, this commit also uses that
new data to much more easily populate the list of scheduled topics in a
group's sidebar, which previously required a subquery and windowing.

tildes/alembic/versions/468cf81f4a6b_topic_schedule_add_latest_topic_id.py

@@ -0,0 +1,103 @@
+"""topic_schedule: add latest_topic_id
+
+Revision ID: 468cf81f4a6b
+Revises: 4d86b372a8db
+Create Date: 2020-06-25 02:53:09.435947
+
+"""
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision = "468cf81f4a6b"
+down_revision = "4d86b372a8db"
+branch_labels = None
+depends_on = None
+
+
+def upgrade():
+    op.add_column(
+        "topic_schedule", sa.Column("latest_topic_id", sa.Integer(), nullable=True)
+    )
+    op.create_foreign_key(
+        op.f("fk_topic_schedule_latest_topic_id_topics"),
+        "topic_schedule",
+        "topics",
+        ["latest_topic_id"],
+        ["topic_id"],
+    )
+
+    op.execute(
+        """
+        create or replace function update_topic_schedule_latest_topic_id() returns trigger as $$
+        begin
+            if (NEW.schedule_id is not null) then
+                update topic_schedule
+                    set latest_topic_id = (
+                        select topic_id
+                            from topics
+                            where schedule_id = NEW.schedule_id
+                                and is_deleted = false
+                                and is_removed = false
+                            order by created_time desc limit 1)
+                    where schedule_id = NEW.schedule_id;
+            end if;
+
+            -- if it was an update that changed schedule_id, need to update the old schedule's
+            -- latest_topic_id as well (this will probably be extremely uncommon)
+            if (TG_OP = 'UPDATE'
+                    and OLD.schedule_id is not null
+                    and OLD.schedule_id is distinct from NEW.schedule_id) then
+                update topic_schedule
+                    set latest_topic_id = (
+                        select topic_id
+                            from topics
+                            where schedule_id = OLD.schedule_id
+                                and is_deleted = false
+                                and is_removed = false
+                            order by created_time desc limit 1)
+                    where schedule_id = OLD.schedule_id;
+            end if;
+
+            return null;
+        end
+        $$ language plpgsql;
+    """
+    )
+
+    op.execute(
+        """
+        create trigger update_topic_schedule_latest_topic_id_insert
+            after insert on topics
+            for each row
+            when (NEW.schedule_id is not null)
+            execute procedure update_topic_schedule_latest_topic_id();
+    """
+    )
+
+    op.execute(
+        """
+        create trigger update_topic_schedule_latest_topic_id_update
+            after update on topics
+            for each row
+            when ((OLD.schedule_id is not null or NEW.schedule_id is not null)
+                and ((OLD.is_deleted is distinct from NEW.is_deleted)
+                    or (OLD.is_removed is distinct from NEW.is_removed)
+                    or (OLD.schedule_id is distinct from NEW.schedule_id)))
+            execute procedure update_topic_schedule_latest_topic_id();
+    """
+    )
+
+
+def downgrade():
+    op.execute("drop trigger update_topic_schedule_latest_topic_id_update on topics")
+    op.execute("drop trigger update_topic_schedule_latest_topic_id_insert on topics")
+    op.execute("drop function update_topic_schedule_latest_topic_id")
+
+    op.drop_constraint(
+        op.f("fk_topic_schedule_latest_topic_id_topics"),
+        "topic_schedule",
+        type_="foreignkey",
+    )
+    op.drop_column("topic_schedule", "latest_topic_id")

tildes/sql/init/triggers/topics/topic_schedule.sql

@@ -0,0 +1,53 @@
+-- Copyright (c) 2020 Tildes contributors <code@tildes.net>
+-- SPDX-License-Identifier: AGPL-3.0-or-later
+
+create or replace function update_topic_schedule_latest_topic_id() returns trigger as $$
+begin
+    if (NEW.schedule_id is not null) then
+        update topic_schedule
+            set latest_topic_id = (
+                select topic_id
+                    from topics
+                    where schedule_id = NEW.schedule_id
+                        and is_deleted = false
+                        and is_removed = false
+                    order by created_time desc limit 1)
+            where schedule_id = NEW.schedule_id;
+    end if;
+
+    -- if it was an update that changed schedule_id, need to update the old schedule's
+    -- latest_topic_id as well (this will probably be extremely uncommon)
+    if (TG_OP = 'UPDATE'
+            and OLD.schedule_id is not null
+            and OLD.schedule_id is distinct from NEW.schedule_id) then
+        update topic_schedule
+            set latest_topic_id = (
+                select topic_id
+                    from topics
+                    where schedule_id = OLD.schedule_id
+                        and is_deleted = false
+                        and is_removed = false
+                    order by created_time desc limit 1)
+            where schedule_id = OLD.schedule_id;
+    end if;
+
+    return null;
+end
+$$ language plpgsql;
+
+
+create trigger update_topic_schedule_latest_topic_id_insert
+    after insert on topics
+    for each row
+    when (NEW.schedule_id is not null)
+    execute procedure update_topic_schedule_latest_topic_id();
+
+
+create trigger update_topic_schedule_latest_topic_id_update
+    after update on topics
+    for each row
+    when ((OLD.schedule_id is not null or NEW.schedule_id is not null)
+        and ((OLD.is_deleted is distinct from NEW.is_deleted)
+            or (OLD.is_removed is distinct from NEW.is_removed)
+            or (OLD.schedule_id is distinct from NEW.schedule_id)))
+    execute procedure update_topic_schedule_latest_topic_id();

tildes/tildes/models/topic/topic.py

@@ -65,6 +65,9 @@ class Topic(DatabaseModel):
           updates to is_deleted in comments.
         - last_activity_time will be updated by insertions, deletions, and updates to
           is_deleted in comments.
+      Outgoing:
+        - Inserting rows or updating is_deleted/is_removed to change visibility will
+          update topic_schedule.latest_topic_id if the topic has a schedule_id.
       Internal:
         - deleted_time will be set when is_deleted is set to true
     """
@@ -127,6 +130,8 @@ class Topic(DatabaseModel):
     user: User = relationship("User", lazy=False, innerjoin=True)
     group: Group = relationship("Group", innerjoin=True)
 
+    schedule = relationship("TopicSchedule", foreign_keys=[schedule_id])
+
     # Create specialized indexes
     __table_args__ = (
         Index("ix_topics_tags_gist", tags, postgresql_using="gist"),

tildes/tildes/models/topic/topic_schedule.py

@@ -9,7 +9,7 @@ from typing import List, Optional
 from dateutil.rrule import rrule
 from jinja2.sandbox import SandboxedEnvironment
 from sqlalchemy import CheckConstraint, Column, ForeignKey, Integer, Text, TIMESTAMP
-from sqlalchemy.orm import backref, relationship
+from sqlalchemy.orm import relationship
 from sqlalchemy.orm.session import Session
 from sqlalchemy.sql.expression import text
 
@@ -23,7 +23,13 @@ from tildes.schemas.topic import TITLE_MAX_LENGTH
 
 
 class TopicSchedule(DatabaseModel):
-    """Model for scheduled topics (auto-posted, often repeatedly on a schedule)."""
+    """Model for scheduled topics (auto-posted, often repeatedly on a schedule).
+
+    Trigger behavior:
+      Incoming:
+        - latest_topic_id will be set when a new topic is inserted for the schedule,
+          and updated when a topic from the schedule is deleted or removed.
+    """
 
     __tablename__ = "topic_schedule"
 
@@ -46,11 +52,11 @@ class TopicSchedule(DatabaseModel):
         TIMESTAMP(timezone=True), nullable=True, index=True
     )
     recurrence_rule: Optional[rrule] = Column(RecurrenceRule, nullable=True)
+    latest_topic_id: int = Column(Integer, ForeignKey("topics.topic_id"), nullable=True)
 
     group: Group = relationship("Group", innerjoin=True)
     user: Optional[User] = relationship("User")
-
-    topics: List[Topic] = relationship(Topic, backref=backref("schedule"))
+    latest_topic: Topic = relationship("Topic", foreign_keys=[latest_topic_id])
 
     def __init__(
         self,

tildes/tildes/views/topic.py

@@ -15,7 +15,8 @@ from pyramid.request import Request
 from pyramid.response import Response
 from pyramid.view import view_config
 from sqlalchemy import cast
-from sqlalchemy.sql.expression import any_, desc, text
+from sqlalchemy.orm import joinedload
+from sqlalchemy.sql.expression import any_, desc
 from sqlalchemy_utils import Ltree
 from webargs.pyramidparser import use_kwargs
 
@@ -249,25 +250,21 @@ def get_group_topics(  # noqa
 
     if isinstance(request.context, Group):
         # Get the most recent topic from each scheduled topic in this group
-        # I'm not even going to attempt to write this query in pure SQLAlchemy
-        topic_id_subquery = """
-            SELECT topic_id FROM (SELECT topic_id, schedule_id, row_number() OVER
-            (PARTITION BY schedule_id ORDER BY created_time DESC) AS rownum FROM topics)
-            AS t WHERE schedule_id IS NOT NULL AND rownum = 1
-        """
-        most_recent_scheduled_topics = (
-            request.query(Topic)
-            .join(TopicSchedule)
+        group_schedules = (
+            request.query(TopicSchedule)
+            .options(joinedload(TopicSchedule.latest_topic))
             .filter(
-                Topic.topic_id.in_(text(topic_id_subquery)),  # type: ignore
                 TopicSchedule.group == request.context,
                 TopicSchedule.next_post_time != None,  # noqa
             )
             .order_by(TopicSchedule.next_post_time)
             .all()
         )
+        most_recent_scheduled_topics = [
+            schedule.latest_topic for schedule in group_schedules
+        ]
     else:
-        most_recent_scheduled_topics = None
+        most_recent_scheduled_topics = []
 
     if is_home_page:
         financial_data = get_financial_data(request.db_session)