Question

I’m building my first django app. I have a user, and the user has a list of favourites. A user has exactly one list of favourites, and that list belongs exclusively to that user.

class User(models.Model):
    name = models.CharField(max_length=200)

class FavouriteList(models.Model):
    user = models.OneToOneField(User)
    favourites = models.ManyToManyField(Favourite, blank=True)

When a new user is created, I want to ensure that the user has a FavouriteList. I’ve looked around in the Django documentation and haven’t had much luck.

Does anyone know how I can ensure that a model has a child object (e.g. FavouriteList) when it is created?

Answer 1

The most common way to accomplish this is to use the Django signals system. You can attach a signal handler (just a function somewhere) to the post_save signal for the User model, and create your favorites list inside of that callback.

from django.db.models.signals import post_save
from django.dispatch import receiver
from django.contrib.auth.models import User

@receiver(post_save, sender=User)
def create_favorites(sender, instance, created, **kwargs):
    if created:
        Favorites.objects.create(user=instance)

The above was adapted from the Django signals docs. Be sure to read the signals docs entirely because there are a few issues that can snag you such as where your signal handler code should live and how to avoid duplicate handlers.