PERF: Remove category_user lookup when loading private messages.

Private messages do not belong to categories so the query is unnecessary
overhead.

app/models/topic_list.rb

@@ -80,7 +80,6 @@ class TopicList < DraftableList
 
     # Attach some data for serialization to each topic
     @topic_lookup = TopicUser.lookup_for(@current_user, @topics) if @current_user
-    @category_user_lookup = CategoryUser.lookup_for(@current_user, @topics.map(&:category_id).uniq) if @current_user
 
     post_action_type =
       if @current_user
@@ -111,7 +110,10 @@ class TopicList < DraftableList
 
     @topics.each do |ft|
       ft.user_data = @topic_lookup[ft.id] if @topic_lookup.present?
-      ft.category_user_data = @category_user_lookup[ft.category_id] if @category_user_lookup.present?
+
+      if ft.regular? && category_user_lookup.present?
+        ft.category_user_data = @category_user_lookup[ft.category_id]
+      end
 
       if ft.user_data && post_action_lookup && actions = post_action_lookup[ft.id]
         ft.user_data.post_action_data = { post_action_type => actions }
@@ -140,4 +142,16 @@ class TopicList < DraftableList
   def attributes
     { 'more_topics_url' => page }
   end
+
+  private
+
+  def category_user_lookup
+    @category_user_lookup ||= begin
+      if @current_user
+        CategoryUser.lookup_for(@current_user, @topics.map(&:category_id).uniq)
+      else
+        []
+      end
+    end
+  end
 end

spec/models/topic_list_spec.rb

@@ -48,6 +48,16 @@ describe TopicList do
     end
   end
 
+  describe '#load_topics' do
+    it 'loads additional data for serialization' do
+      category_user = CategoryUser.create!(user: user, category: topic.category)
+
+      topic = topic_list.load_topics.first
+
+      expect(topic.category_user_data).to eq(category_user)
+    end
+  end
+
   describe '#top_tags' do
     it 'should return the right tags' do
       tag = Fabricate(:tag, topics: [topic])