PERF: Fix n+1 for categories + featured topics (#16188)

`topic.featured_topic` and `topic.category` are used by `TopicGuardian#can_see_topic?`

app/models/category_list.rb

@@ -62,7 +62,7 @@ class CategoryList
 
     category_featured_topics = CategoryFeaturedTopic.select([:category_id, :topic_id]).order(:rank)
 
-    @all_topics = Topic.where(id: category_featured_topics.map(&:topic_id))
+    @all_topics = Topic.where(id: category_featured_topics.map(&:topic_id)).includes(:shared_draft, :category)
 
     if @guardian.authenticated?
       @all_topics = @all_topics

spec/requests/categories_controller_spec.rb

@@ -159,6 +159,42 @@ describe CategoriesController do
       expect(category_response["subcategory_list"][0]["id"]).to eq(subcategory.id)
     end
 
+    it "does not n+1 with multiple topics" do
+      category1 = Fabricate(:category)
+      category2 = Fabricate(:category)
+      topic1 = Fabricate(:topic, category: category1)
+
+      CategoryFeaturedTopic.feature_topics
+      SiteSetting.desktop_category_page_style = "categories_with_featured_topics"
+
+      # warmup
+      get "/categories.json"
+      expect(response.status).to eq(200)
+
+      first_request_queries = track_sql_queries do
+        get "/categories.json"
+        expect(response.status).to eq(200)
+      end
+
+      category_response = response.parsed_body["category_list"]["categories"].find { |c| c["id"] == category1.id }
+      expect(category_response["topics"].count).to eq(1)
+
+      topic2 = Fabricate(:topic, category: category2)
+      CategoryFeaturedTopic.feature_topics
+
+      second_request_queries = track_sql_queries do
+        get "/categories.json"
+        expect(response.status).to eq(200)
+      end
+
+      category1_response = response.parsed_body["category_list"]["categories"].find { |c| c["id"] == category1.id }
+      category2_response = response.parsed_body["category_list"]["categories"].find { |c| c["id"] == category2.id }
+      expect(category1_response["topics"].size).to eq(1)
+      expect(category2_response["topics"].size).to eq(1)
+
+      expect(first_request_queries.count).to eq(second_request_queries.count)
+    end
+
     it 'does not show uncategorized unless allow_uncategorized_topics' do
       SiteSetting.desktop_category_page_style = "categories_boxes_with_topics"
 

spec/support/helpers.rb

@@ -181,4 +181,17 @@ module Helpers
     target.send(:remove_const, const)
     target.const_set(const, old)
   end
+
+  def track_sql_queries
+    queries = []
+    callback = ->(*, payload) {
+      queries << payload.fetch(:sql) unless ["CACHE", "SCHEMA"].include?(payload.fetch(:name))
+    }
+
+    ActiveSupport::Notifications.subscribed(callback, "sql.active_record") do
+      yield
+    end
+
+    queries
+  end
 end