My pre_save signal isn't applying my string generator - django

Question

I wrote a simple string generator for my order_id field.
I tested the generator script in shell, and it works perfectly.
But when I run the server, and try to create an order in django admin, the order id field remains empty when I click save.
What am I doing wrong?
from datetime import date
from django.db import models
from django.db.models.signals import pre_save
from cartapp.models import Cart
class Order(models.Model):
order_id = models.CharField(max_length=120)
cart = models.ForeignKey(Cart, on_delete=models.CASCADE)
status = models.CharField(max_length=50, default='Waiting', null=True, blank=True)
order_total = models.DecimalField(default=0.0, max_digits=10, decimal_places=1)
date_created = models.DateTimeField(auto_now_add=True)
def order_id_generator(instance):
today = date.today().strftime("%Y-%m-%d")
last_order_raw = Order.objects.latest('order_id').date_created
last_order_date = str(last_order_raw).split(' ')[0]
if today != last_order_date:
new_order_id = str(today + " 1")
else:
last_order = Order.objects.latest('order_id')
extract = last_order.order_id.split(' ')[1]
increment = int(extract) + 1
new_order_id = today + " " + str(increment)
return new_order_id
def pre_save_order_id(sender, instance, *args, **kwargs):
if not instance.order_id:
instance.order_id = order_id_generator(instance)
pre_save.connect(pre_save_order_id, sender=Order)

Answer 1

I noticed that you are passing instance to order_id_generator but doesn't use it there. You can avoid using signals and you can use your function as the model field default:
class Order(models.Model):
order_id = models.CharField(max_length=120, default=order_id_generator)
and you doesn't need an arg instance in your function:
def order_id_generator():
today = date.today().strftime("%Y-%m-%d")
last_order_raw = Order.objects.latest('order_id').date_created
last_order_date = str(last_order_raw).split(' ')[0]
if today != last_order_date:
new_order_id = str(today + " 1")
else:
last_order = Order.objects.latest('order_id')
extract = last_order.order_id.split(' ')[1]
increment = int(extract) + 1
new_order_id = today + " " + str(increment)
return new_order_id