class Foo(models.Model):
    secret = models.CharField(max_length=12, blank=True, editable=False)

    def generateSecret(self):
        s = struct.pack('L', random.getrandbits(32))
        s += struct.pack('L', random.getrandbits(32))
        s += struct.pack('L', random.getrandbits(8))
        self.secret = base64.urlsafe_b64encode(s[0:9])
        self.save()