Update authenticatable.rb

lib/devise/models/authenticatable.rb

@@ -206,8 +206,10 @@ module Devise
           :case_insensitive_keys, :http_authenticatable, :params_authenticatable, :skip_session_storage,
           :http_authentication_key)
 
-
         def serialize_into_session(record)
+          # This is a work around a mongoid bug, that's why we do
+          # the whole *record.to_key dance. Hopefully we can remove
+          # this once mongoid generates proper to_key ids.
           [[*record.to_key].first.to_s, record.authenticatable_salt]
         end