Django 2.2 user = models.ForeignKey(User, null=True, blank=True) TypeError: __init__() missing 1 required positional argument: 'on_delete'

Question

I am trying to create a user object in my Cart class in my models.py file. And I am receiving the following error:

user        = models.ForeignKey(User, null=True, blank=True)
TypeError: __init__() missing 1 required positional argument: 'on_delete'

Can you help me add the required positional argument: on_delete in a proper way in oder to use the user object later while creating a cart session?

Thanks in advance.

In my carts app models.py I have the following code:

from django.conf import settings 

from django.db import models
from django.db.models.signals import pre_save, post_save, m2m_changed

from products.models import Product


User = settings.AUTH_USER_MODEL

class CartManager(models.Manager):
    def new_or_get(self, request):
        cart_id = request.session.get("cart_id", None)
        qs = self.get_queryset().filter(id=cart_id)
        if qs.count() == 1:
            new_obj = False
            cart_obj = qs.first()
            if request.user.is_authenticated and cart_obj.user is None:
                cart_obj.user = request.user
                cart_obj.save()
        else:
            cart_obj = Cart.objects.new(user=request.user)
            new_obj = True  
            request.session['card_id'] = cart_obj.id
        return cart_obj, new_obj

    def new(self, user=None):
        user_obj = None
        if user is not None:
            if user.is_authenticated:
                user_obj = user
        return self.model.objects.create(user=user_obj)


class Cart(models.Model):
    user        = models.ForeignKey(User, null=True, blank=True)
    products    = models.ManyToManyField(Product, blank=True)
    subtotal    = models.DecimalField(default=0.00, max_digits=100, decimal_places=2)
    total       = models.DecimalField(default=0.00, max_digits=100, decimal_places=2)
    updated     = models.DateTimeField(auto_now=True)
    timestamp   = models.DateTimeField(auto_now_add=True)

    objects = CartManager()

    def __str__(self):
        return str(self.id)


def m2m_changed_cart_receiver(sender, instance, action, *args, **kwargs):
    if action == 'post_add' or action == 'post_remove' or action == 'post_clear':
        products = instance.products.all()
        total = 0
        for x in products:
            total += x.price
        if instance.subtotal !=total: 
            instance.subtotal = total
            instance.save()

m2m_changed.connect(m2m_changed_cart_receiver, sender=Cart.products.through)

def pre_save_cart_receiver(sender, instance, *args, **kwargs):
    if instance.subtotal > 0:
        instance.total = instance.subtotal 
    else:
        instance.total = 0.00

pre_save.connect(pre_save_cart_receiver, sender=Cart)




   Thanks,

Answer 1

When working with ForeignKey class, on_delete parameter is required in __init__. (It should be set explicitly) in Django(2.0 for example). It was not required in Django(1.8).

from django.conf import settings 
from django.db import models
from django.db.models.signals import pre_save, post_save, m2m_changed

from products.models import Product


User = settings.AUTH_USER_MODEL

class CartManager(models.Manager):
    def new_or_get(self, request):
        cart_id = request.session.get("cart_id", None)
        qs = self.get_queryset().filter(id=cart_id)
        if qs.count() == 1:
            new_obj = False
            cart_obj = qs.first()
            if request.user.is_authenticated and cart_obj.user is None:
                cart_obj.user = request.user
                cart_obj.save()
        else:
            cart_obj = Cart.objects.new(user=request.user)
            new_obj = True  
            request.session['card_id'] = cart_obj.id
        return cart_obj, new_obj

    def new(self, user=None):
        user_obj = None
        if user is not None:
            if user.is_authenticated:
                user_obj = user
        return self.model.objects.create(user=user_obj)


class Cart(models.Model):
    user        = models.ForeignKey(User, null=True, blank=True, on_delete=models.CASCADE) # You should add on_delete=models.CASCADE argument
    products    = models.ManyToManyField(Product, blank=True)
    subtotal    = models.DecimalField(default=0.00, max_digits=100, decimal_places=2)
    total       = models.DecimalField(default=0.00, max_digits=100, decimal_places=2)
    updated     = models.DateTimeField(auto_now=True)
    timestamp   = models.DateTimeField(auto_now_add=True)

    objects = CartManager()

    def __str__(self):
        return str(self.id)


def m2m_changed_cart_receiver(sender, instance, action, *args, **kwargs):
    if action == 'post_add' or action == 'post_remove' or action == 'post_clear':
        products = instance.products.all()
        total = 0
        for x in products:
            total += x.price
        if instance.subtotal !=total: 
            instance.subtotal = total
            instance.save()

m2m_changed.connect(m2m_changed_cart_receiver, sender=Cart.products.through)

def pre_save_cart_receiver(sender, instance, *args, **kwargs):
    if instance.subtotal > 0:
        instance.total = instance.subtotal 
    else:
        instance.total = 0.00

pre_save.connect(pre_save_cart_receiver, sender=Cart)

Answer 2

You are missing one required argument in the user field that is needed in ForeignKey

you can add this

on_delete=models.SET_NULL

or

on_delete=models.CASCADE

then the field will be

user = models.ForeignKey(User, null=True, related_name="author",
    on_delete=models.SET_NULL)

or

user = models.ForeignKey(User, null=True, blank=True,
    on_delete=models.CASCADE)

you can get more information about on_delete parameter at offical docs link