Shopping cart implementation for Django

import json
from collections import OrderedDict
from django.core.urlresolvers import reverse
from django.core.exceptions import ObjectDoesNotExist, MultipleObjectsReturned
from .models import Product


class CartItem:
    def __init__(self, product_pk, quantity):
        product = Product.objects.get(pk=product_pk)
        photo = product.productphoto_set.first()
        self.pk = product.pk
        self.url = reverse('product-detail', args=[product.category.slug, product.slug])
        self.name = product.name
        self.photo_url = photo.photo.url if photo else None
        self.price = product.sale_price if product.sale_price else product.price
        self.quantity = quantity
        self.subtotal_price = self.price*self.quantity

    @property
    def serializable(self):
        return self.__dict__


class Cart:
    def __init__(self, session, session_key='cart'):
        self._session = session
        self._session_key = session_key
        if session_key in session:
            self._items = json.loads(session[session_key], object_pairs_hook=OrderedDict)
        else:
            self._items = OrderedDict()
            self.update_session()

    def change_quantity(self, product_pk, quantity_diff):
        if product_pk not in self._items:
            self._items[product_pk] = 0
        self._items[product_pk] += quantity_diff
        if self._items[product_pk] < 1:
            self.remove(product_pk)

    def remove(self, product_pk):
        self._items.pop(product_pk, None)

    def clear(self):
        self._items = OrderedDict()

    def update_session(self):
        self._session[self._session_key] = json.dumps(self._items)

    @property
    def items(self):
        for product_pk, quantity in self._items.items():
            try:
                yield CartItem(product_pk, quantity)
            except (ObjectDoesNotExist, MultipleObjectsReturned):
                pass

    @property
    def serializable(self):
        cart = {'items': [], 'total_quantity': 0, 'total_price': 0}
        for item in self.items:
            cart['items'].append(item.serializable)
            cart['total_quantity'] += item.quantity
            cart['total_price'] += item.subtotal_price
        return cart