User ⊆ Principal ∧ Principal ⊄ User

I.e. calling Member#principal will always work.
On the other hand calling #user will fail if
the member is a group.

app/models/member.rb

@@ -46,7 +46,7 @@ class Member < ActiveRecord::Base
   after_destroy :unwatch_from_permission_change
 
   def name
-    user.name
+    principal.name
   end
 
   def to_s