How to tell if a model instance is new or not when using UUIDField as a Primary Key

Question

I have a model that requires some post-processing (I generate an MD5 of the body field).

models.py

class MyModel(models.Model):
    id = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)
    body = models.TextField()
    md5 = models.CharField(max_length=32)
    ...

    def save(self, *args, **kwargs):
        if self.pk is None: # Only for create (edit disabled)
            self.md5 = get_md5(self.body)
            super(MyModel, self).save(*args, **kwargs)

The problem is that the final block won't execute because I don't see a way to check if the instance is new or not: self.pk is never None because a UUID is populated before saving.

I'd like to know what the best practice is for handling this.

Thanks in advance.

Update:

The only solution I can think of is to call the database directly and:

1. Check if the id exists
2. Compare the modified and created fields to tell if it's an edit

Answer 1

Looks like the cleanest approach to this is to make sure that all your models have a created date on them by inheriting from an Abstract model, then you simply check if created has a value:

models.py

class BaseModel(models.Model):
    """
    Base model which all other models can inherit from.
    """
    id = fields.CustomUUIDField(primary_key=True, default=uuid.uuid4, editable=False)
    created = models.DateTimeField(auto_now_add=True)
    modified = models.DateTimeField(auto_now=True)

    class Meta:
        # Abstract models are not created in the DB
        abstract = True

class MyModel(BaseModel):
    my_field = models.CharField(max_length=50)

    def save(self, *args, **kwargs):
        if self.created:
            # Do stuff
            pass
        super(MyModel, self).save(*args, **kwargs)