Essa classe tem o método new_or_get() que cria e retorna uma Order, se não existir nenhum pedido com essa associação de billing_profile e carrinho, caso já exista um Order com essa associação, ele só retorna esse pedido já existente.

django_ecommerce/e_commerce/orders/models.py


import math
from django.db import models
from django.db.models.signals import pre_save, post_save
from billing.models import BillingProfile
from carts.models import Cart
from e_commerce.utils import unique_order_id_generator

ORDER_STATUS_CHOICES = (
    ('created', 'Criado'),
    ('paid', 'Pago'),
    ('shipped', 'Enviado'),
    ('refunded', 'Devolvido'),
)
class OrderManager(models.Manager):
    def new_or_get(self, billing_profile, cart_obj):
        created = False
        qs = self.get_queryset().filter(
                billing_profile = billing_profile, 
                cart = cart_obj, 
                active = True)
        if qs.count() == 1:
            obj = qs.first()
        else:
            obj = self.model.objects.create(
                    billing_profile = billing_profile, 
                    cart=cart_obj)
            created = True
        return obj, created

class Order(models.Model):
    billing_profile = models.ForeignKey(BillingProfile, on_delete=models.CASCADE, null = True, blank = True)
    order_id = models.CharField(max_length = 120, blank = True)
    # billing_profile = ?
    # shipping_address = ?
    # billing_address
    cart = models.ForeignKey(Cart, on_delete=models.CASCADE, null = True)
    status = models.CharField(max_length = 120, default = 'created', choices = ORDER_STATUS_CHOICES )
    shipping_total = models.DecimalField(default = 5.99, max_digits = 100, decimal_places = 2)
    total = models.DecimalField(default = 0.00, max_digits = 100, decimal_places = 2)
    active = models.BooleanField(default=True)

    def __str__(self):
        return self.order_id

    objects = OrderManager()

    def update_total(self):
        cart_total = self.cart.total
        shipping_total = self.shipping_total
        new_total = math.fsum([cart_total, shipping_total])
        formatted_total = format(new_total, '.2f')
        self.total = formatted_total 
        self.save()
        return new_total

def pre_save_create_order_id(sender, instance, *args, **kwargs):
    if not instance.order_id:
        instance.order_id = unique_order_id_generator(instance)
    # retorna todos os orders com esta instância de cart, 
    # excluindo aqueles que têm a mesma instância de billing_profile
    qs = Order.objects.filter(cart = instance.cart).exclude(billing_profile = instance.billing_profile)
    if qs.exists():
        qs.update = False

pre_save.connect(pre_save_create_order_id, sender = Order)

def post_save_cart_total(sender, instance, created, *args, **kwargs):
    if not created:
        cart_obj = instance
        cart_total = cart_obj.total
        cart_id = cart_obj.id
        qs = Order.objects.filter(cart__id=cart_id)
        if qs.count() == 1:
            order_obj = qs.first()
            order_obj.update_total()

post_save.connect(post_save_cart_total, sender=Cart)

def post_save_order(sender, instance, created, *args, **kwargs):
    print("Executando")
    if created:
        print("Atualizando")
        instance.update_total()

post_save.connect(post_save_order, sender=Order)

No views.py do carts, coloque as alterações em azul.

O que esse código faz é verificar se existe um billing_profile para o usuário, para que seja possível atribuir ou criar um Order associando ao seu Cart.

Django_ecommerce/e_commerce/carts/views.py


from django.shortcuts import render, redirect

from accounts.forms import LoginForm, GuestForm
from accounts.models import GuestEmail
from billing.models import BillingProfile
from orders.models import Order
from products.models import Product
from .models import Cart

def cart_home(request):
    cart_obj, new_obj  = Cart.objects.new_or_get(request)
    return render(request, "carts/home.html", {"cart": cart_obj})

def cart_update(request):
    print(request.POST)
    product_id = request.POST.get('product_id')
    if product_id is not None:
        try:
            product_obj = Product.objects.get(id = product_id)
        except Product.DoesNotExist:
            print("Mostrar mensagem ao usuário, esse produto acabou!")
            return redirect("cart:home")
        cart_obj, new_obj = Cart.objects.new_or_get(request) 
        if product_obj in cart_obj.products.all(): 
            cart_obj.products.remove(product_obj) 
        else: 
            cart_obj.products.add(product_obj)
        request.session['cart_items'] = cart_obj.products.count()
    return redirect("cart:home")

def checkout_home(request):
    #aqui a gente pega o carrinho
    cart_obj, cart_created= Cart.objects.new_or_get(request)
    order_obj = None
    #se o carrinho acabou de ser criado, ele tá zerado
    #ou se o carrinho já existir mas não tiver nada dentro
    if cart_created or cart_obj.products.count() == 0:
        return redirect("cart:home")
    #aqui a order associada ao carrinho
    user = request.user
    billing_profile = None
    login_form = LoginForm()
    guest_form = GuestForm()
    guest_email_id = request.session.get('guest_email_id')
    if user.is_authenticated:
        billing_profile, billing_profile_created = BillingProfile.objects.get_or_create(
            user=user, email=user.email)
    elif guest_email_id is not None:
        guest_email_obj = GuestEmail.objects.get(id=guest_email_id)
        billing_profile, billing_guest_profile_created = BillingProfile.objects.get_or_create(
            email=guest_email_obj.email)
    else:
        pass

    if billing_profile is not None:
        order_obj, order_obj_created = Order.objects.new_or_get(billing_profile, cart_obj)

    context = {
        "object": order_obj,
        "billing_profile": billing_profile,
        "login_form": login_form,
        "guest_form": guest_form
    }
    return render(request, "carts/checkout.html", context)