From 3700514819d362e9758da121e988005320f24d0c Mon Sep 17 00:00:00 2001
From: Bianca Nenciu <nbianca@users.noreply.github.com>
Date: Mon, 25 Sep 2023 19:38:54 +0300
Subject: [PATCH] DEV: Prefer nested queries (#23464)

Some sites have a large number of categories and fetching the category
IDs or category topic IDs just to build another query can take a long
time or resources (i.e. memory).
---
 app/controllers/about_controller.rb       | 2 +-
 app/controllers/application_controller.rb | 2 +-
 app/controllers/topics_controller.rb      | 2 +-
 app/models/category_featured_topic.rb     | 3 +--
 app/models/topic.rb                       | 3 +--
 5 files changed, 5 insertions(+), 7 deletions(-)

diff --git a/app/controllers/about_controller.rb b/app/controllers/about_controller.rb
index ba5d59879e1..002f2e52b06 100644
--- a/app/controllers/about_controller.rb
+++ b/app/controllers/about_controller.rb
@@ -20,7 +20,7 @@ class AboutController < ApplicationController
     unless current_user.staff?
       RateLimiter.new(current_user, "live_post_counts", 1, 10.minutes).performed!
     end
-    category_topic_ids = Category.pluck(:topic_id).compact!
+    category_topic_ids = Category.select(:topic_id).where.not(topic_id: nil)
     public_topics =
       Topic.listable_topics.visible.secured(Guardian.new(nil)).where.not(id: category_topic_ids)
     stats = { public_topic_count: public_topics.count }
diff --git a/app/controllers/application_controller.rb b/app/controllers/application_controller.rb
index 4b8b060dc72..92bdf13da01 100644
--- a/app/controllers/application_controller.rb
+++ b/app/controllers/application_controller.rb
@@ -931,7 +931,7 @@ class ApplicationController < ActionController::Base
         Discourse
           .cache
           .fetch(key, expires_in: 10.minutes) do
-            category_topic_ids = Category.pluck(:topic_id).compact
+            category_topic_ids = Category.select(:topic_id).where.not(topic_id: nil)
             @top_viewed =
               TopicQuery
                 .new(nil, except_topic_ids: category_topic_ids)
diff --git a/app/controllers/topics_controller.rb b/app/controllers/topics_controller.rb
index 06eb231ff46..1a0588dd689 100644
--- a/app/controllers/topics_controller.rb
+++ b/app/controllers/topics_controller.rb
@@ -344,7 +344,7 @@ class TopicsController < ApplicationController
     topic = Topic.find_by(id: params[:id])
     guardian.ensure_can_edit!(topic)
 
-    category = Category.where(id: params[:category_id].to_i).first
+    category = Category.find_by(id: params[:category_id].to_i)
     guardian.ensure_can_publish_topic!(topic, category)
 
     row_count = SharedDraft.where(topic_id: topic.id).update_all(category_id: category.id)
diff --git a/app/models/category_featured_topic.rb b/app/models/category_featured_topic.rb
index 1602dc7ad09..5192f3f4a6a 100644
--- a/app/models/category_featured_topic.rb
+++ b/app/models/category_featured_topic.rb
@@ -29,8 +29,7 @@ class CategoryFeaturedTopic < ActiveRecord::Base
 
     if batched
       if categories.length == batch_size
-        next_id =
-          Category.where("id > ?", categories.last.id).order("id asc").limit(1).pluck(:id)[0]
+        next_id = Category.where("id > ?", categories.last.id).order(:id).pick(:id)
         next_id ? Discourse.redis.setex(NEXT_CATEGORY_ID_KEY, 1.day, next_id) : clear_batch!
       else
         clear_batch!
diff --git a/app/models/topic.rb b/app/models/topic.rb
index e2c6434c2ba..4ed8755aa65 100644
--- a/app/models/topic.rb
+++ b/app/models/topic.rb
@@ -569,8 +569,7 @@ class Topic < ActiveRecord::Base
     topics = topics.limit(opts[:limit]) if opts[:limit]
 
     # Remove category topics
-    category_topic_ids = Category.pluck(:topic_id).compact!
-    topics = topics.where("topics.id NOT IN (?)", category_topic_ids) if category_topic_ids.present?
+    topics = topics.where.not(id: Category.select(:topic_id).where.not(topic_id: nil))
 
     # Remove muted and shared draft categories
     remove_category_ids =