- Avoid using .first since it will create an additional query.

- Handle scenario where @posts is empty.

actionpack/lib/action_view/helpers/atom_feed_helper.rb

@@ -32,7 +32,7 @@ module AtomFeedHelper
       #   app/views/posts/index.atom.builder:
       #     atom_feed do |feed|
       #       feed.title("My great blog!")
-      #       feed.updated(@posts.first.created_at)
+      #       feed.updated(@posts[0].created_at) if @posts.length > 0
       #
       #       @posts.each do |post|
       #         feed.entry(post) do |entry|