diff --git a/lib/mongoid/tree.rb b/lib/mongoid/tree.rb
index 682469c..3d8fb49 100644
--- a/lib/mongoid/tree.rb
+++ b/lib/mongoid/tree.rb
@@ -1,3 +1,5 @@
+require 'active_support/concern'
+
 module Mongoid
   ##
   # = Mongoid::Tree