Django FileField with upload_to determined at runtime

gistfile1.py

def content_file_name(instance, filename):
    return '/'.join(['content', instance.user.username, filename])

class Content(models.Model):
    name = models.CharField(max_length=200)
    user = models.ForeignKey(User)
    file = models.FileField(upload_to=content_file_name)

taken from here: http://stackoverflow.com/questions/1190697/django-filefield-with-upload-to-determined-at-runtime