Django: Access primary key in models.filefield(upload_to) location.

Question

I'd like to save my files using the primary key of the entry.

Here is my code:

def get_nzb_filename(instance, filename):
    if not instance.pk:
        instance.save() # Does not work.
    name_slug = re.sub('[^a-zA-Z0-9]', '-', instance.name).strip('-').lower()
    name_slug = re.sub('[-]+', '-', name_slug)
    return u'files/%s_%s.nzb' % (instance.pk, name_slug)

class File(models.Model):
    nzb = models.FileField(upload_to=get_nzb_filename)
    name = models.CharField(max_length=256)

I know the first time an object is saved the primary key isn't available, so I'm willing to take the extra hit to save the object just to get the primary key, and then continue on.

The above code doesn't work. It throws the following error:

maximum recursion depth exceeded while calling a Python object

I'm assuming this is an infinite loop. Calling the save method would call the get_nzb_filename method, which would again call the save method, and so on.

I'm using the latest version of the Django trunk.

How can I get the primary key so I can use it to save my uploaded files?

---

Update @muhuk:

I like your solution. Can you help me implement it? I've updated my code to the following and the error is 'File' object has no attribute 'create'. Perhaps I'm using what you've written out of context?

def create_with_pk(self):
    instance = self.create()
    instance.save()
    return instance

def get_nzb_filename(instance, filename):
    if not instance.pk:
        create_with_pk(instance)
    name_slug = re.sub('[^a-zA-Z0-9]', '-', instance.name).strip('-').lower()
    name_slug = re.sub('[-]+', '-', name_slug)
    return u'files/%s_%s.nzb' % (instance.pk, name_slug)

class File(models.Model):
    nzb = models.FileField(upload_to=get_nzb_filename, blank=True, null=True)
    name = models.CharField(max_length=256)

Instead of enforcing the required field in my model I'll do it in my Form class. No problem.

Answer 1

It seems you'll need to pre-generate your File models with empty file fields first. Then pick up one and save it with the given file object.

You can have a custom manager method like this;

def create_with_pk(self):
    instance = self.create()
    instance.save()     # probably this line is unneeded
    return instance

But this will be troublesome if either of your fields is required. Because you are initially creating a null object, you can't enforce required fields on the model level.

EDIT

create_with_pk is supposed to be a custom manager method, in your code it is just a regular method. Hence self is meaningless. It is all properly documented with examples.