Django - automatically create a model instance when another model instance is created

Question

I have an overrided User model and a Cart model. I expect that a Cart model instance is created automatically once a User model instance is created. I am trying to pass the newly registered user into the get_queryset method, but no idea how to do it. Are there any other better ways of doing this? It's because I may need to do the same thing for other models unlike the User model which has a form that can pass values to the get_queryset method.

account/models.py:

class Cart(models.Model):

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

    def __str__(self):
        return self.user.email + '_cart'

account/views.py:

class RegisterView(generic.CreateView):
    template_name = 'account/register.html'
    form_class = RegisterForm
    success_url = reverse_lazy('book:home')

    def get_queryset(self):
        sign_up = self.request.POST.get('register')
        if sign_up:
            c = Cart.objects.create(user=???)
            c.save()

account/templates/account/register.html:

<form name="register" method="post">{% csrf_token %}
    {{ form.as_p }}
    <input type="submit" value="Sign Up">
</form>

JPG · Accepted Answer · 2018-09-06 04:26:22Z

23

Method-1: Use Django's post_save signal (as @AKX said)

from django.contrib.auth import get_user_model


@receiver(post_save, sender=get_user_model())
def create_user_cart(sender, instance, created, **kwargs):
    if created:
        Cart.objects.create(user=instance)

Method-2: Override the save() method of your User model or extended Usermodel

class MyUserModel(....):
    # your code
    def save(self,*args,**kwargs):
        created = not self.pk
        super().save(*args,**kwargs)
        if created:
            Cart.objects.create(user=self)

edited Sep 6, 2018 at 4:26

answered Sep 6, 2018 at 4:21

JPG

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3 Comments

Luis Rodriguez Over a year ago

Hello @JPG, i would like to know what is this created = not self.pk doing! i am little bit confused in here, this method worked for me but i would like to know how is this code actually working!

JPG Over a year ago

@LuisRodriguez self.pk will be None for instances that are going to be created.

Matt Sanders Over a year ago

@LuisRodriguez pk is Django's reliable shortcut for the primary key field. Basically what the code is doing checking if the primary key was set before save, if so this is a new object creation, otherwise it is an update.

AKX · Accepted Answer · 2018-09-06 04:11:37Z

8

Use a Django post_save signal hooked up to the User model.

See the signals tutorial for more information.

answered Sep 6, 2018 at 4:11

AKX

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Comments

Achuth Varghese · Accepted Answer · 2020-08-16 10:07:14Z

2

override save method, signals are harder to read/track through multiple files and are synchronous.

https://lincolnloop.com/blog/django-anti-patterns-signals/

class Pizza(models.Model):
    has_pepperoni = models.BooleanField(default=False)

    def save(self, *args, **kwargs):
        created = self.pk is None
        super(Pizza, self).save(*args, **kwargs)
        if created and self.has_pepperoni:
            ToppingSales.objects.filter(name='pepperoni').update(
                units_sold=F('units_sold') + 1)           


class ToppingSales(models.Model):
    name = models.CharField(max_length=100, unique=True)
    units_sold = models.PositiveIntegerField(default=0)

edited Aug 16, 2020 at 10:07

Achuth Varghese

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answered Nov 19, 2019 at 5:45

jcirni

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Comments

Rob · Accepted Answer · 2020-09-01 23:49:15Z

0

I believe the modern ways (as per the docs) are to do this during a create() class method or to write a custom manager.

https://docs.djangoproject.com/en/3.1/ref/models/instances/

answered Sep 1, 2020 at 23:49

Rob

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