Checking if a object exists on save in Django

Having used integer primary keys for many projects, I've only recently switched over to using UUIDs on a few projects about a little less than a year ago. In the switch, an old solution I've often used for checking if an object exists, or is being created, has become obsolete.

A typical solution for checking if an object is being created in the save method is usually to check if self.pk is None as such:

This isn't always true (see below)

class Parrot(UUIDModel):
    is_loud = models.BooleanField(default=True)
    ...

    def save(self, *args, **kwargs):
        if self.pk is None:
            do_some_stuff()
        super(Parrot, self).save(*args, **kwargs)

The problem

The issue with the above solution is that we are relying on a "hack" that utilizes Django's cached attributes, that checks to see if the primary key is None. This will certainly fail if you're using UUIDs or any primary key that sets the default parameter on the field.

Ex:

import uuid


class UUIDModel(models.Model):
    id = models.UUIDField(primary_key=True, editable=False, default=uuid.uuid4)
    ...

    class Meta:
        abstract = True

Solutions

1. One solution would be to add a created timestamp field to the model and check if created is not None instead. But this is potentially the same issue as above if a default value is ever set on the created field.

2. Another solution would be to perform a query to check if the object exists. Ex:

class Parrot(UUIDModel):
    is_loud = models.BooleanField(default=True)
    ...

    def save(self, *args, **kwargs):
        try:
            parrot = Parrot.objects.get(id=parrot.pk)
        except Parrot.DoesNotExist:
            parrot = None
        if parrot is None:
            do_some_stuff()
        super(Parrot, self).save(*args, **kwargs)

The downside with this solution is that you are now performing an extra query just to check if the object exists. But barring any possible race condition situations or n+1 query scenario, this is probably your best bet for checking if the object exists.

If anyone else has a solution feel free to contact me, I'd love to hear your thoughts.

Sources:

• Stack Overflow Answer highlighting the partially correct solution

• Stack Overflow Answer highlighting the issue with using self.pk is None