如何保存空模型（我只需要创建pk）

I have these models:

我有这些模型：

class Storage(models.Model):
    id = models.AutoField(primary_key=True)

    def __unicode__(self):
        return str(self.id)

class StorageType(models.Model):
    tripa = 'Tripa'
    portada = 'Portada'
    type_choice = (
        (tripa, 'Tripa'),
        (portada, 'Portada'),
    )
    sto_type = models.CharField(max_length=9, choices=type_choice, default=tripa)
    storage = models.ForeignKey(Storage)
    paper_type = models.ForeignKey(Paper)
    paper_qnty = models.IntegerField(blank=True, default=0)
    web_paper_qnty = models.IntegerField(blank=True, default=0)

    def __unicode__(self):
        return '%s of %s' %(self.sto_type, str(self.storage))
class Storage(