[Django]-Save model field value as default for another field

The problem is that you set the index before saving the instance, at that point, new model instance do not yet have an id. So you can perform two .save()s: one before obtaining the id, and one to save the new index.

def save(self, *args, **kwargs):
    if self.id is None:
        super().save(*args, **kwargs)
    if self.index is None:
        self.index = self.id
        super().save(*args, **kwargs)

But that being said, it is still not a good idea: there are various ways to circumvent the .save(..) method, for example when updating specific fields, etc.

Therefore I advice you to make a column _index instead that is NULL-able, and then write a @property to handle the case where the index is NULL:

class PhotoManager(models.Model):
    id = models.AutoField(primary_key=True)
    _index = models.PositiveIntegerField(null=True, default=None, blank=True)

    @property
    def index(self):
        if self._index is not None:
            return self._index
        return self.id