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

app/models/post.rb

@@ -19,6 +19,7 @@ class Post < ActiveRecord::Base
 
   belongs_to :user
   belongs_to :topic, counter_cache: :posts_count
+  belongs_to :reply_to_user, class_name: "User"
 
   has_many :post_replies
   has_many :replies, through: :post_replies
@@ -190,12 +191,6 @@ class Post < ActiveRecord::Base
     (quote_count == 0) && (reply_to_post_number.present?)
   end
 
-  # Get the post that we reply to.
-  def reply_to_user
-    return if reply_to_post_number.blank?
-    Post.where(topic_id: topic_id, post_number: reply_to_post_number).first.try(:user)
-  end
-
   def reply_notification_target
     return if reply_to_post_number.blank?
     Post.where("topic_id = :topic_id AND post_number = :post_number AND user_id <> :user_id",
@@ -268,8 +263,14 @@ class Post < ActiveRecord::Base
     PostRevisor.new(self).revise!(updated_by, new_raw, opts)
   end
 
+
+  # TODO: move into PostCreator
   # Various callbacks
   before_create do
+    if reply_to_post_number.present?
+      self.reply_to_user_id ||= Post.select(:user_id).where(topic_id: topic_id, post_number: reply_to_post_number).first.try(:user_id) 
+    end
+
     self.post_number ||= Topic.next_post_number(topic_id, reply_to_post_number.present?)
     self.cooked ||= cook(raw, topic_id: topic_id)
     self.sort_order = post_number
@@ -278,6 +279,7 @@ class Post < ActiveRecord::Base
   end
 
   # TODO: Move some of this into an asynchronous job?
+  # TODO: Move into PostCreator
   after_create do
     # Update attributes on the topic - featured users and last posted.
     attrs = {last_posted_at: created_at, last_post_user_id: user_id}

app/models/user.rb

@@ -402,6 +402,11 @@ class User < ActiveRecord::Base
     admin? || TrustLevel.compare(trust_level, level)
   end
 
+  # a touch faster than automatic
+  def admin? 
+    admin
+  end
+
   def change_trust_level(level)
     raise "Invalid trust level #{level}" unless TrustLevel.valid_level?(level)
     self.trust_level = TrustLevel.levels[level]

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

lib/topic_view.rb

@@ -34,6 +34,7 @@ class TopicView
 
     @all_posts = @posts
 
+
     filter_posts(options)
 
     @draft_key = @topic.draft_key
@@ -130,7 +131,8 @@ class TopicView
     @max = post_number - 1
 
     @posts = @posts.reverse_order.where("post_number < ?", post_number)
-    @posts = @posts.includes(:topic).joins(:user).limit(SiteSetting.posts_per_page)
+
+    @posts = @posts.includes(:reply_to_user).includes(:topic).joins(:user).limit(SiteSetting.posts_per_page)
     @min = @max - @posts.size
     @min = 1 if @min < 1
   end
@@ -140,7 +142,7 @@ class TopicView
     @initial_load = false
     @min = post_number
     @posts = @posts.regular_order.where("post_number > ?", post_number)
-    @posts = @posts.includes(:topic).joins(:user).limit(SiteSetting.posts_per_page)
+    @posts = @posts.includes(:reply_to_user).includes(:topic).joins(:user).limit(SiteSetting.posts_per_page)
     @max = @min + @posts.size
   end
 
@@ -275,7 +277,7 @@ class TopicView
 
   def filter_posts_in_range(min, max)
     @min, @max = min, max
-    @posts = @posts.where("post_number between ? and ?", @min, @max).includes(:user).regular_order
+    @posts = @posts.where("post_number between ? and ?", @min, @max).includes(:user).includes(:reply_to_user).regular_order
   end
 
   def find_topic(topic_id)