cache reply_to_user_id in post to eliminate a very nasty n+1 query

db/migrate/20130319122248_add_reply_to_user_id_to_post.rb

@@ -0,0 +1,16 @@
+class AddReplyToUserIdToPost < ActiveRecord::Migration
+  def up
+    # caching this column makes the topic page WAY faster
+    add_column :posts, :reply_to_user_id, :integer
+    execute 'UPDATE posts p SET reply_to_user_id = (
+                SELECT u.id from users u 
+                JOIN posts p2 ON  p2.user_id = u.id AND 
+                                  p2.post_number = p.reply_to_post_number AND
+                                  p2.topic_id = p.topic_id
+            )'
+  end
+
+  def down
+    remove_column :posts, :reply_to_user_id
+  end
+end