PERF: Fix N+1 queries problem when listing topics list (#20971)

This performance regression was introduced in
7c6a8f1c74 where the preloading of tags in
`TopicQuery` was accidentally removed.

lib/topic_query.rb

@@ -721,7 +721,11 @@ class TopicQuery
       end
     end
 
-    result = filter_by_tags(result)
+    if SiteSetting.tagging_enabled
+      result = result.includes(:tags)
+      result = filter_by_tags(result)
+    end
+
     result = apply_ordering(result, options)
 
     all_listable_topics =
@@ -1151,8 +1155,6 @@ class TopicQuery
   end
 
   def filter_by_tags(result)
-    return result if !SiteSetting.tagging_enabled
-
     tags_arg = @options[:tags]
 
     if tags_arg && tags_arg.size > 0

spec/requests/list_controller_spec.rb

@@ -106,6 +106,50 @@ RSpec.describe ListController do
       expect(first_item.css('[itemprop="url"]')[0]["href"]).to eq(topic.url)
     end
 
+    it "does not result in N+1 queries when topics have tags and tagging_enabled site setting is enabled" do
+      SiteSetting.tagging_enabled = true
+      tag = Fabricate(:tag)
+      topic.tags << tag
+
+      # warm up
+      get "/latest.json"
+      expect(response.status).to eq(200)
+
+      initial_sql_queries_count =
+        track_sql_queries do
+          get "/latest.json"
+
+          expect(response.status).to eq(200)
+
+          body = response.parsed_body
+
+          expect(body["topic_list"]["topics"].map { |t| t["id"] }).to contain_exactly(topic.id)
+          expect(body["topic_list"]["topics"][0]["tags"]).to contain_exactly(tag.name)
+        end.count
+
+      tag2 = Fabricate(:tag)
+      topic2 = Fabricate(:topic, tags: [tag2])
+
+      new_sql_queries_count =
+        track_sql_queries do
+          get "/latest.json"
+
+          expect(response.status).to eq(200)
+
+          body = response.parsed_body
+
+          expect(body["topic_list"]["topics"].map { |t| t["id"] }).to contain_exactly(
+            topic.id,
+            topic2.id,
+          )
+
+          expect(body["topic_list"]["topics"][0]["tags"]).to contain_exactly(tag2.name)
+          expect(body["topic_list"]["topics"][1]["tags"]).to contain_exactly(tag.name)
+        end.count
+
+      expect(new_sql_queries_count).to eq(initial_sql_queries_count)
+    end
+
     it "does not N+1 queries when topic featured users have different primary groups" do
       user.update!(primary_group: group)