comments: minimize sql queries.

Certain parts of comment rendering triggered sql queries that we didn't
really need to do. Rework things to avoid this.

* Preload comment creators in order to display commenter names with link_to_user.

* Preload comment votes in order to display "undo vote" links. Only preload
  votes for members since anonymous users can't vote and don't have "undo
  vote" links.

* Rework various conditionals to do the filtering in Ruby so that we
  avoid issuing any extra queries in sql.

* Avoid issuing any queries at all when the post doesn't have any
  comments (when last_commented_at is blank).

app/controllers/comments_controller.rb

@@ -74,7 +74,10 @@ private
 
   def index_by_post
     @posts = Post.where("last_comment_bumped_at IS NOT NULL").tag_match(params[:tags]).reorder("last_comment_bumped_at DESC NULLS LAST").paginate(params[:page], :limit => 5, :search_count => params[:search])
-    @posts.each # hack to force rails to eager load
+
+    @posts = @posts.includes(comments: [:creator])
+    @posts = @posts.includes(comments: [:votes]) if CurrentUser.is_member?
+
     respond_with(@posts) do |format|
       format.xml do
         render :xml => @posts.to_xml(:root => "posts")

app/controllers/posts_controller.rb

@@ -23,6 +23,10 @@ class PostsController < ApplicationController
   def show
     @post = Post.find(params[:id])
 
+    @comments = @post.last_commented_at.present? ? @post.comments : Comment.none
+    @comments = @comments.includes(:creator)
+    @comments = @comments.includes(:votes) if CurrentUser.is_member?
+
     include_deleted = @post.is_deleted? || (@post.parent_id.present? && @post.parent.is_deleted?) || CurrentUser.user.show_deleted_children?
     @parent_post_set = PostSets::PostRelationship.new(@post.parent_id, :include_deleted => include_deleted)
     @children_post_set = PostSets::PostRelationship.new(@post.id, :include_deleted => include_deleted)