It took me many hours to find out why my sessions are often gone when using memcache session storage. Then I remember, that I cannot simply store any objects to a memcache session storage! In the older rails, we have to do:

model :cart

to store an object of class Cart. In the newer rails, just replace 'model' with 'require_dependency'. For more info, read this site under If you have an “uninitialized constant” error