I've seen pk is used all over the models.py file except in get_product. This generates errors if the product has not an id field (but another primary key). The proposed patch solves the problem.
|migrations||adding South migrations to the project and tests|
|cart.py||add items summary method|
|models.py||support for primary keys fields other than id|
|tests.py||Added more info in README, AUTHOR and tests|