Django get_or_create on reverse relationship - django

Question

I'd like to use get_or_create instead of doing this:
cart = client.cart
cartitems_cart = cart.__cartitem_set
if not cartitems_cart.filter(item=stockitem).exists():
cartitem = CartItem.objects.create(item=stockitem)
else:
cartitem = cartitems_cart.objects.get(item=stockitem)
is it possible somehow?
The models look like this:
class CartItem(models.Model):
item = models.ForeignKey(StockItem, blank=True)
quantity = models.IntegerField(default=1, null=True)
class Cart(models.Model):
items = models.ManyToManyField(CartItem, blank=True)
So, I need to get or create a CartItem that is related with a Cart. I'm not sure how to write that query.

Answer 1

First off, your design is flawed. M2M relation allows a cart item to have more than one cart when it shouldn't. You should add a FK field to CartItem model instead of adding a M2M field to Cart model:
class Cart(models.Model):
user = models.ForeignKey(User)
class CartItem(models.Model):
item = models.ForeignKey(StockItem, blank=True)
quantity = models.IntegerField(default=1, null=True)
cart = models.ForeignKey(Cart, related_name="items")
so you can do the following:
cartitem, created = client.cart.items.get_or_create(item=stockitem)
or
cartitem, created = CartItem.objects.get_or_create(cart=client.cart, item=stockitem)
Please look at the documentation for more information regarding what other parameters get_or_create takes.