diff --git a/guides/rails_guides/helpers.rb b/guides/rails_guides/helpers.rb
index a288d0f0f4..760b196abd 100644
--- a/guides/rails_guides/helpers.rb
+++ b/guides/rails_guides/helpers.rb
@@ -17,7 +17,7 @@ module RailsGuides
     end
 
     def documents_flat
-      documents_by_section.map {|section| section['documents']}.flatten
+      documents_by_section.flat_map {|section| section['documents']}
     end
 
     def finished_documents(documents)