diff --git a/lib/acts_as_tree.rb b/lib/acts_as_tree.rb
index c933dbc..fbb6b22 100644
--- a/lib/acts_as_tree.rb
+++ b/lib/acts_as_tree.rb
@@ -121,7 +121,7 @@ def self.root
         end
 
         def self.roots
-          where(:#{configuration[:foreign_key]} => nil).default_tree_order
+          where.not(:id => where(:parent => all)).default_tree_order
         end
       EOV