PERF: Fix N+1 queries problem on topic view page (#19629)

`User#flair_group` was not preloaded leading to the N+1 queries problem
when multiple users have flair groups.

lib/topic_view.rb

@@ -477,7 +477,7 @@ class TopicView
   def participants
     @participants ||= begin
       participants = {}
-      User.where(id: post_counts_by_user.keys).includes(:primary_group).each { |u| participants[u.id] = u }
+      User.where(id: post_counts_by_user.keys).includes(:primary_group, :flair_group).each { |u| participants[u.id] = u }
       participants
     end
   end
@@ -774,7 +774,7 @@ class TopicView
   def filter_posts_by_ids(post_ids)
     @posts = Post.where(id: post_ids, topic_id: @topic.id)
       .includes(
-        { user: :primary_group },
+        { user: [:primary_group, :flair_group] },
         :reply_to_user,
         :deleted_by,
         :incoming_email,

spec/requests/topics_controller_spec.rb

@@ -2068,6 +2068,54 @@ RSpec.describe TopicsController do
       expect(response.status).to eq(200)
     end
 
+    it 'does not result in N+1 queries problem when multiple topic participants have primary or flair group configured' do
+      user2 = Fabricate(:user)
+      user3 = Fabricate(:user)
+      post2 = Fabricate(:post, topic: topic, user: user2)
+      post3 = Fabricate(:post, topic: topic, user: user3)
+      group = Fabricate(:group)
+      user2.update!(primary_group: group)
+      user3.update!(flair_group: group)
+
+      # warm up
+      get "/t/#{topic.id}.json"
+      expect(response.status).to eq(200)
+
+      first_request_queries = track_sql_queries do
+        get "/t/#{topic.id}.json"
+
+        expect(response.status).to eq(200)
+
+        expect(response.parsed_body["details"]["participants"].map { |u| u["id"] }).to contain_exactly(
+          post_author1.id,
+          user2.id,
+          user3.id
+        )
+      end
+
+      group2 = Fabricate(:group)
+      user4 = Fabricate(:user, flair_group: group2)
+      user5 = Fabricate(:user, primary_group: group2)
+      post4 = Fabricate(:post, topic: topic, user: user4)
+      post5 = Fabricate(:post, topic: topic, user: user5)
+
+      second_request_queries = track_sql_queries do
+        get "/t/#{topic.id}.json"
+
+        expect(response.status).to eq(200)
+
+        expect(response.parsed_body["details"]["participants"].map { |u| u["id"] }).to contain_exactly(
+          post_author1.id,
+          user2.id,
+          user3.id,
+          user4.id,
+          user5.id
+        )
+      end
+
+      expect(second_request_queries.count).to eq(first_request_queries.count)
+    end
+
     context 'when a topic with nil slug exists' do
       before do
         nil_slug_topic = Fabricate(:topic)