How can I auto-fill a field in the model without showing a form field to the user?

I'm new to django, but coming from php I think its the greatest thing ever.

I have a model for Invoices ...

{{{

from django.conf import settings

from django.db import models

from django.core.urlresolvers import reverse

#from django.contrib.auth.models import User

# Create your models here.

class Invoice(models.Model):

    created_at = models.DateTimeField(auto_now_add=True)

    their_company = models.CharField(max_length=255)

    invoice_number = models.CharField(max_length=50, default='')

    bill_to = models.CharField(max_length=255, default='')

    created_by = models.ForeignKey(settings.AUTH_USER_MODEL)

    

    class Meta:

        ordering = ['invoice_number', ]

    

    def __str__(self):

        return self.invoice_number

    

    def get_absolute_url(self):

        return reverse("accounting:invoices:detail", kwargs={"pk": self.pk})

}}}

right now the user has to put in the invoice number themselves. I want to not give the user that option though - I'd like to autofill that field in the database with a concatenation of the 2 digit year (17) plus 2 digit month (07) plus 2 digit day (24) and then a 2 digit auto increment for every invoice created on that specific date so the first invoice created today would be 17072401 and the second would be 17072402, etc. I'm guessing I need a field just for the date and another for the iteration of each invoice.

Can anyone give me some direction on this? First I'm not sure how to autofill a field in the database without getting the input from the user, and I'm also not sure how I would do the concatenation with the right numbers. Thanks for your help

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