PERF: Avoid running the same query twice in `TopicViewSerializer#details`.

app/models/topic.rb

@@ -446,6 +446,7 @@ class Topic < ActiveRecord::Base
     @post_numbers = nil
     @public_topic_timer = nil
     @private_topic_timer = nil
+    @is_category_topic = nil
     super(options)
   end
 
@@ -1347,6 +1348,10 @@ SQL
     private_messages.with_subtype(topic_subtype).where('topics.created_at >= ? AND topics.created_at <= ?', start_date, end_date).group('date(topics.created_at)').order('date(topics.created_at)').count
   end
 
+  def is_category_topic?
+    @is_category_topic ||= Category.exists?(topic_id: self.id.to_i)
+  end
+
   private
 
   def update_category_topic_count_by(num)

lib/guardian/topic_guardian.rb

@@ -86,15 +86,15 @@ module TopicGuardian
   def can_delete_topic?(topic)
     !topic.trashed? &&
     is_staff? &&
-    !(Category.exists?(topic_id: topic.id)) &&
+    !(topic.is_category_topic?) &&
     !Discourse.static_doc_topic_ids.include?(topic.id)
   end
 
   def can_convert_topic?(topic)
     return false unless SiteSetting.enable_personal_messages?
     return false if topic.blank?
-    return false if topic && topic.trashed?
-    return false if Category.where("topic_id = ?", topic.id).exists?
+    return false if topic.trashed?
+    return false if topic.is_category_topic?
     return true if is_admin?
     is_moderator? && can_create_post?(topic)
   end