Django - How to get self.id when saving a new object?

I have a problem in one of my models. I'm uploading an image, and I want to store the id (pk in the database table) but I need to know at which point Django will have access to self.id.

models.py

class BicycleAdItemKind(MPTTModel):
    def url(self, filename):
        pdb.set_trace()

        url = "MultimediaData/HelpAdImages/ItemKind/%s/%s" % (self.id, filename)
        return url

    def item_kind_image(self):
        return '<img align="middle" src="/media/%s" height="60px" />' % self.image
    item_kind_image.allow_tags = True     

    # Bicicleta completa, Componentes para bicicleta, Acessorios para ciclista
    n_item_kind      = models.CharField(max_length=50) 
    parent           = TreeForeignKey('self', null=True,
                                      blank=True, related_name='children')
    description      = models.TextField(null=True, blank=True)
    image            = models.ImageField(upload_to=url, null=True, blank=True)
    date_inserted    = models.DateTimeField(auto_now_add=True)
    date_last_update = models.DateTimeField(auto_now=True)

    def __unicode__(self):
        return self.n_item_kind

    class MPTTMeta:
        order_insertion_by = ['n_item_kind']

The problem is in the url() method; I can only get self.id when updating an object, I don't get the self.id when creating a new object. How can I modify this model so that I get self.id when creating a new object?

With the current code, when I'm creating a new object I will end up with a url like:

MultimediaData/HelpAdImages/ItemKind/None/somefile.jpg

And I need to have something like:

MultimediaData/HelpAdImages/ItemKind/35/somefile.jpg

Any clues?

If it's a new object, you need to save it first and then access self.id, because

"There's no way to tell what the value of an ID will be before you call save(), 
 because that value is calculated by your database, not by Django."

Check django's document https://docs.djangoproject.com/en/dev/ref/models/instances/