post archives: fix N+1 problem when fetching previous version.

app/controllers/post_versions_controller.rb

@@ -3,7 +3,7 @@ class PostVersionsController < ApplicationController
   respond_to :html, :xml, :json
 
   def index
-    @post_versions = PostArchive.includes(:post, :updater).search(params[:search]).order("updated_at desc, id desc").paginate(params[:page], :limit => params[:limit], :search_count => params[:search])
+    @post_versions = PostArchive.includes(:updater, post: [:versions]).search(params[:search]).order("updated_at desc, id desc").paginate(params[:page], :limit => params[:limit], :search_count => params[:search])
     respond_with(@post_versions) do |format|
       format.xml do
         render :xml => @post_versions.to_xml(:root => "post-versions")

app/controllers/reports_controller.rb

@@ -21,6 +21,6 @@ class ReportsController < ApplicationController
 
   def upload_tags
     @user = User.find(params[:user_id])
-    @upload_reports = Reports::UploadTags.includes(versions: [:post]).for_user(params[:user_id]).order("id desc").paginate(params[:page], :limit => params[:limit])
+    @upload_reports = Reports::UploadTags.includes(versions: { post: :versions }).for_user(params[:user_id]).order("id desc").paginate(params[:page], :limit => params[:limit])
   end
 end

app/models/post_archive.rb

@@ -96,7 +96,13 @@ class PostArchive < ActiveRecord::Base
   end
 
   def previous
-    PostArchive.where("post_id = ? and version < ?", post_id, version).order("version desc").first
+    # HACK: if all the post versions for this post have already been preloaded,
+    # we can use that to avoid a SQL query.
+    if association(:post).loaded? && post.association(:versions).loaded?
+      post.versions.sort_by(&:version).reverse.find { |v| v.version < version }
+    else
+      PostArchive.where("post_id = ? and version < ?", post_id, version).order("version desc").first
+    end
   end
 
   def diff(version = nil)