+ t.add :writable_by
+ end
+
+ def maybe_invalidate_permissions_cache
+ if uuid_changed? or owner_uuid_changed?
+ # This can change users' permissions on other groups as well as
+ # this one.
+ invalidate_permissions_cache
+ end
+ end
+
+ def invalidate_permissions_cache
+ # Ensure a new group can be accessed by the appropriate users
+ # immediately after being created.
+ User.invalidate_permissions_cache