remove N+1 queries

app/models/category.rb

@@ -1,5 +1,9 @@
 class Category < ActiveRecord::Base
   belongs_to :topic, dependent: :destroy
+  belongs_to :topic_only_relative_url, 
+    select: "id, title",
+    class_name: "Topic",
+    foreign_key: "topic_id"
   belongs_to :user
 
   has_many :topics
@@ -43,7 +47,7 @@ class Category < ActiveRecord::Base
   end
 
   def topic_url
-    topic.try(:relative_url)
+    topic_only_relative_url.try(:relative_url)
   end
 
   before_save do

app/models/site.rb

@@ -22,7 +22,7 @@ class Site
   end
 
   def categories
-    Category.popular
+    Category.popular.includes(:topic_only_relative_url)
   end
 
   def archetypes

lib/topic_query.rb

@@ -196,7 +196,7 @@ class TopicQuery
         end
       end
 
-      result = result.listable_topics.includes(:category)
+      result = result.listable_topics.includes(:category => :topic_only_relative_url)
       result = result.where('categories.name is null or categories.name <> ?', query_opts[:exclude_category]) if query_opts[:exclude_category]
       result = result.where('categories.name = ?', query_opts[:only_category]) if query_opts[:only_category]
       result = result.limit(page_size) unless query_opts[:limit] == false