title: Increase by one per user per submit
body: <p>I got the models shown down below.</p>
  <p>I want that my invoice_id field gets the value 000001 when user A submits to it and as well 000001 when user B submits to it. It should auto increase from there, so if user A stays idle but user B makes other submits, B's next will get 000002, 000003 etc. If user C then joins and submit, he should get 000001 too. How would I approach this?</p>
  <p>my models:</p>
  <pre><code>class Account(AbstractBaseUser):
    email = models.EmailField(verbose_name='email', max_length=60, unique=True)
    username = models.CharField(max_length=30, unique=True)
    first_name = models.CharField(max_length=25)
    last_name = models.CharField(max_length=45)
    date_joined = models.DateTimeField(verbose_name='date joined', auto_now_add=True)
    last_login = models.DateTimeField(verbose_name='last login', auto_now=True)
    is_active = models.BooleanField(default=True)
    is_admin = models.BooleanField(default=False)
    is_staff = models.BooleanField(default=False)
    is_superuser = models.BooleanField(default=False)
    # Company Profile Fields
    co_logo = models.ImageField(upload_to=&quot;dashboard/logos&quot;, blank=True)
    co_name = models.CharField(max_length=45)
    co_address = models.CharField(max_length=45)
    co_zip = models.CharField(max_length=45)
    co_city = models.CharField(max_length=45)
  
    objects = MyAccountManager()
  
    USERNAME_FIELD = 'email'
    REQUIRED_FIELDS = ['username']
  
    def __str__(self):
      return self.email
  
    def has_perm(self, perm, obj=None):
      return self.is_admin
  
    def has_module_perms(self, app_label):
      return True
  
  
  class Invoice(models.Model):
    client = models.ForeignKey(BusinessContact, default=None, on_delete=models.CASCADE)
    user = models.ForeignKey(Account, default=None, on_delete=models.CASCADE)
    invoice_id = models.CharField(unique=True, max_length=6, null=True,
                                  blank=True, editable=False)
    place = models.CharField(max_length=40)
    date_created = models.DateTimeField(verbose_name='date created', auto_now_add=True)
    date_sent = models.DateTimeField(verbose_name='date sent by mail', auto_now=True)
    date_deadline = models.DateTimeField(verbose_name='payment deadline', auto_now=False)
    bank_number = models.CharField(max_length=40)
    iban_number = models.CharField(max_length=40)
    swift_bic = models.CharField(max_length=40)
    total = models.CharField(max_length=40)
    currency = models.CharField(max_length=40)
    paid_status = models.BooleanField(default=False)
    custom_comment = models.CharField(max_length=40)
    reverse_charge = models.BooleanField(default=False)
  
    def save(self, *args, **kwargs):
      if self.pk:
        self.invoice_id += 1
      return super(Invoice, self).save(*args, **kwargs)
  
    def __str__(self):
      return str(self.user) if self.user else ''
  </code></pre>