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

@@ -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 }

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)

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)