Attach the django-shop cart to a User that started shopping anonymously

models.py

# add this soewhere where it gets loaded very early, i.e. 
# your shop's models.py
from django.contrib.auth import login
from django.contrib.auth.signals import user_logged_in
from django.dispatch import receiver

from registration.signals import user_activated
from shop.models.defaults.cart import Cart


@receiver(user_activated)
def login_user(sender, **kwargs):
    user = kwargs.get('user')
    user.backend = 'django.contrib.auth.backends.ModelBackend'
    login(kwargs.get('request'), user)


def attach_cart(sender, **kwargs):
    user = kwargs.get('user')
    user.backend = 'django.contrib.auth.backends.ModelBackend'
    session = kwargs.get('request').session
    cart_id = session.get('cart_id')
    if cart_id:
        # If the user logged in before, he left another cart bound to his user,
        # we have to delete this old cart first.
        cart = None

        try:
            old_cart = Cart.objects.get(user=user)
            new_cart = Cart.objects.get(id=cart_id)
            # If the new cart is empty and the old cart was not, we will keep
            # the old cart.
            if (new_cart.items.all().count() == 0
                    and old_cart.items.all().count() > 0):
                new_cart.delete()
                cart = old_cart
            else:
                old_cart.delete()
                cart = new_cart
        except Cart.DoesNotExist:
            cart = Cart.objects.get(id=cart_id)

        cart.user_id = user.id
        cart.save()
    if not user.is_authenticated():
        login(kwargs.get('request'), user)


user_activated.connect(attach_cart)
user_logged_in.connect(attach_cart)