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

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)