Diff of https://gist.github.com/kyanny/cb3cfb759dd0fc8c0ec8e0ad175bfd47...https://gist.github.com/kyanny/42d961b252a5f1aca797f8482d959ca5

mm.rb.diff

diff --git a/mongomapper-inherit-one/mm.rb b/mongomapper-inherit-one/mm.rb
index b7cf1e5..5b727a2 100644
--- a/mongomapper-inherit-one/mm.rb
+++ b/mongomapper-inherit-one/mm.rb
@@ -29,6 +29,7 @@ class Membership
 end
 
 class WrapUser < User
+  one :membership, foreign_key: :user_id
 end
 
 wrap_user = WrapUser.create!