I´m about to use an admin.py-file to register the models. within this file, I´ll use a raw_id_field ... currently, you use models.py to register the models for the admin-interface. I can send you admin.py once I´m finished.
remove admin.site.register from models.py
add admin.py (see below)
from my point of view, it´s good to have admin.py anway, since people may want to customize the admin-behaviour of piston.