FEATURE: experiment with hot sort order (#25274)

This introduces a new experimental hot sort ordering. 

It attempts to float top conversations by first prioritizing a  topics with lots of recent activity (likes and users responding) 

The schedule that updates hot topics is disabled unless the hidden site setting: `experimental_hot_topics` is enabled. 

You can control "decay" with `hot_topic_gravity` and `recency` with `hot_topics_recent_days` 

Data is stored in the new `topic_hot_scores` table and you can check it out on the `/hot` route once 
enabled. 
---------

Co-authored-by: Penar Musaraj <pmusaraj@gmail.com>

app/controllers/list_controller.rb

@@ -261,6 +261,14 @@ class ListController < ApplicationController
     render "list", formats: [:rss]
   end
 
+  def hot_feed
+    discourse_expires_in 1.minute
+
+    @topic_list = TopicQuery.new(nil).list_hot
+
+    render "list", formats: [:rss]
+  end
+
   def category_feed
     guardian.ensure_can_see!(@category)
     discourse_expires_in 1.minute

app/jobs/scheduled/update_topic_hot_scores.rb

@@ -0,0 +1,11 @@
+# frozen_string_literal: true
+
+module Jobs
+  class UpdateTopicHotScores < ::Jobs::Scheduled
+    every 10.minutes
+
+    def execute(args)
+      TopicHotScore.update_scores if SiteSetting.experimental_hot_topics
+    end
+  end
+end

app/models/topic_hot_score.rb

@@ -0,0 +1,140 @@
+# frozen_string_literal: true
+
+class TopicHotScore < ActiveRecord::Base
+  belongs_to :topic
+
+  DEFAULT_BATCH_SIZE = 1000
+
+  def self.update_scores(max = DEFAULT_BATCH_SIZE)
+    # score is
+    # (total likes - 1) / (age in hours + 2) ^ gravity
+
+    # 1. insert a new record if one does not exist (up to batch size)
+    # 2. update recently created (up to batch size)
+    # 3. update all top scoring topics (up to batch size)
+
+    now = Time.zone.now
+
+    args = {
+      now: now,
+      gravity: SiteSetting.hot_topics_gravity,
+      max: max,
+      private_message: Archetype.private_message,
+      recent_cutoff: now - SiteSetting.hot_topics_recent_days.days,
+    }
+
+    # insert up to BATCH_SIZE records that are missing from table
+    DB.exec(<<~SQL, args)
+      INSERT INTO topic_hot_scores (
+        topic_id,
+        score,
+        recent_likes,
+        recent_posters,
+        created_at,
+        updated_at
+      )
+      SELECT
+        topics.id,
+        0.0,
+        0,
+        0,
+        :now,
+        :now
+
+      FROM topics
+      LEFT OUTER JOIN topic_hot_scores ON topic_hot_scores.topic_id = topics.id
+      WHERE topic_hot_scores.topic_id IS NULL
+        AND topics.deleted_at IS NULL
+        AND topics.archetype <> :private_message
+        AND topics.created_at <= :now
+      ORDER BY topics.bumped_at desc
+      LIMIT :max
+    SQL
+
+    # update recent counts for batch
+    DB.exec(<<~SQL, args)
+      UPDATE topic_hot_scores thsOrig
+      SET
+          recent_likes = COALESCE(new_values.likes_count, 0),
+          recent_posters = COALESCE(new_values.unique_participants, 0),
+          recent_first_bumped_at = COALESCE(new_values.first_bumped_at, ths.recent_first_bumped_at)
+      FROM
+        topic_hot_scores ths
+        LEFT OUTER JOIN
+        (
+          SELECT
+              t.id AS topic_id,
+              COUNT(DISTINCT p.user_id) AS unique_participants,
+              (
+                SELECT COUNT(*)
+                FROM post_actions pa
+                JOIN posts p2 ON p2.id = pa.post_id
+                WHERE p2.topic_id = t.id
+                  AND pa.post_action_type_id = 2 -- action_type for 'like'
+                  AND pa.created_at >= :recent_cutoff
+                  AND pa.deleted_at IS NULL
+              ) AS likes_count,
+              MIN(p.created_at) AS first_bumped_at
+          FROM
+              topics t
+          JOIN
+              posts p ON t.id = p.topic_id
+          WHERE
+              p.created_at >= :recent_cutoff
+              AND t.archetype <> 'private_message'
+              AND t.deleted_at IS NULL
+              AND p.deleted_at IS NULL
+              AND t.created_at <= :now
+              AND t.bumped_at >= :recent_cutoff
+              AND p.created_at < :now
+              AND p.created_at >= :recent_cutoff
+          GROUP BY
+              t.id
+        ) AS new_values
+      ON ths.topic_id = new_values.topic_id
+
+      WHERE thsOrig.topic_id = ths.topic_id
+    SQL
+
+    # update up to BATCH_SIZE records that are out of date based on age
+    # we need an extra index for this
+    DB.exec(<<~SQL, args)
+      UPDATE topic_hot_scores ths
+      SET score = topics.like_count /
+        (EXTRACT(EPOCH FROM (:now - topics.created_at)) / 3600 + 2) ^ :gravity
+ +
+        CASE WHEN ths.recent_first_bumped_at IS NULL THEN 0 ELSE
+          (ths.recent_likes + ths.recent_posters) /
+          (EXTRACT(EPOCH FROM (:now - recent_first_bumped_at)) / 3600 + 2) ^ :gravity
+        END
+        ,
+        updated_at = :now
+
+      FROM topics
+      WHERE topics.id IN (
+        SELECT topic_id FROM topic_hot_scores
+        ORDER BY score DESC, recent_first_bumped_at DESC NULLS LAST
+        LIMIT :max
+      ) AND ths.topic_id = topics.id
+    SQL
+  end
+end
+
+# == Schema Information
+#
+# Table name: topic_hot_scores
+#
+#  id                     :bigint           not null, primary key
+#  topic_id               :integer          not null
+#  score                  :float            default(0.0), not null
+#  recent_likes           :integer          default(0), not null
+#  recent_posters         :integer          default(0), not null
+#  recent_first_bumped_at :datetime
+#  created_at             :datetime         not null
+#  updated_at             :datetime         not null
+#
+# Indexes
+#
+#  index_topic_hot_scores_on_score_and_topic_id  (score,topic_id) UNIQUE
+#  index_topic_hot_scores_on_topic_id            (topic_id) UNIQUE
+#

config/locales/client.en.yml

@@ -2971,6 +2971,7 @@ en:
         category: "There are no more %{category} topics."
         tag: "There are no more %{tag} topics."
         top: "There are no more top topics."
+        hot: "There are no more hot topics."
         bookmarks: "There are no more bookmarked topics."
         filter: "There are no more topics."
 
@@ -4059,6 +4060,10 @@ en:
         title: "Unseen"
         lower_title: "unseen"
         help: "new topics and topics you are currently watching or tracking with unread posts"
+      hot:
+        title: "Hot"
+        lower_title: "hot"
+        help: "top recent topics"
       new:
         lower_title_with_count:
           one: "%{count} new"

config/routes.rb

@@ -1217,6 +1217,7 @@ Discourse::Application.routes.draw do
 
     get "latest.rss" => "list#latest_feed", :format => :rss
     get "top.rss" => "list#top_feed", :format => :rss
+    get "hot.rss" => "list#hot_feed", :format => :rss
 
     Discourse.filters.each { |filter| get "#{filter}" => "list##{filter}" }
 

config/site_settings.yml

@@ -3111,3 +3111,12 @@ dashboard:
   verbose_user_stat_count_logging:
     hidden: true
     default: false
+  experimental_hot_topics:
+    hidden: true
+    default: false
+  hot_topics_gravity:
+    hidden: true
+    default: 1.8
+  hot_topics_recent_days:
+    hidden: true
+    default: 7

db/migrate/20240116043702_create_topic_hot_scores.rb

@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+class CreateTopicHotScores < ActiveRecord::Migration[7.0]
+  def change
+    create_table :topic_hot_scores do |t|
+      t.integer :topic_id, null: false
+      t.float :score, null: false, default: 0.0
+      t.integer :recent_likes, null: false, default: 0
+      t.integer :recent_posters, null: false, default: 0
+      t.datetime :recent_first_bumped_at
+      t.timestamps
+    end
+
+    add_index :topic_hot_scores, :topic_id, unique: true
+    add_index :topic_hot_scores, %i[score topic_id], unique: true
+  end
+end

lib/discourse.rb

@@ -313,11 +313,11 @@ module Discourse
   end
 
   def self.filters
-    @filters ||= %i[latest unread new unseen top read posted bookmarks]
+    @filters ||= %i[latest unread new unseen top read posted bookmarks hot]
   end
 
   def self.anonymous_filters
-    @anonymous_filters ||= %i[latest top categories]
+    @anonymous_filters ||= %i[latest top categories hot]
   end
 
   def self.top_menu_items

lib/topic_query.rb

@@ -338,6 +338,14 @@ class TopicQuery
     create_list(:bookmarks) { |l| l.where("tu.bookmarked") }
   end
 
+  def list_hot
+    create_list(:hot, unordered: true) do |topics|
+      topics.joins("JOIN topic_hot_scores on topics.id = topic_hot_scores.topic_id").order(
+        "topic_hot_scores.score DESC",
+      )
+    end
+  end
+
   def list_top_for(period)
     score_column = TopTopic.score_column_for_period(period)
     create_list(:top, unordered: true) do |topics|

spec/models/topic_hot_scores_spec.rb

@@ -0,0 +1,58 @@
+# frozen_string_literal: true
+
+RSpec.describe TopicHotScore do
+  describe ".update_scores" do
+    fab!(:user)
+    fab!(:user2) { Fabricate(:user) }
+
+    it "can correctly update like counts and post counts and account for activity" do
+      freeze_time
+
+      TopicHotScore.create!(topic_id: -1, score: 0.0, recent_likes: 99, recent_posters: 0)
+
+      old_post = Fabricate(:post, created_at: 10.months.ago)
+      topic = old_post.topic
+
+      new_reply = Fabricate(:post, user: user, topic: topic, created_at: 4.hours.ago)
+      newer_reply = Fabricate(:post, user: user2, topic: topic, created_at: 1.hour.ago)
+      Fabricate(:post, user: user2, topic: topic, created_at: 1.minute.ago)
+
+      freeze_time(1.year.ago)
+      PostActionCreator.like(user, old_post)
+      freeze_time(1.year.from_now)
+
+      PostActionCreator.like(user2, new_reply)
+      PostActionCreator.like(user, newer_reply)
+
+      TopicHotScore.update_scores
+
+      hot_scoring = TopicHotScore.find_by(topic_id: topic.id)
+
+      expect(hot_scoring.recent_likes).to eq(2)
+      expect(hot_scoring.recent_posters).to eq(2)
+      expect(hot_scoring.recent_first_bumped_at).to eq_time(new_reply.created_at)
+      expect(hot_scoring.score).to be_within(0.001).of(1.020)
+
+      expect(TopicHotScore.find_by(topic_id: -1).recent_likes).to eq(0)
+    end
+
+    it "can correctly set scores for topics" do
+      freeze_time
+
+      topic1 = Fabricate(:topic, like_count: 3, created_at: 1.hour.ago)
+      topic2 = Fabricate(:topic, like_count: 10, created_at: 3.hour.ago)
+
+      TopicHotScore.update_scores
+
+      expect(TopicHotScore.find_by(topic_id: topic1.id).score).to be_within(0.001).of(0.415)
+      expect(TopicHotScore.find_by(topic_id: topic2.id).score).to be_within(0.001).of(0.551)
+
+      freeze_time(2.hours.from_now)
+
+      TopicHotScore.update_scores
+
+      expect(TopicHotScore.find_by(topic_id: topic1.id).score).to be_within(0.001).of(0.165)
+      expect(TopicHotScore.find_by(topic_id: topic2.id).score).to be_within(0.001).of(0.301)
+    end
+  end
+end