Auto increment django model field per user

Question

I have this model:

class Invoice(models.Model):
    owner  = models.ForeignKey(settings.AUTH_USER_MODEL)
    data   = models.TextField(default=None, blank=True, null=True)
    number = models.PositiveIntegerField(default=0, null=False)

What I need is to auto-increment the field number for each separated user. The rationale is that each user has a list of Invoice, starting from number=1 to number=latest.number+1.

I do known about F() expressions, but can't figure out how to reference the latest/greatest number for each specific user. Maybe Invoice.objects.filter(owner=request.user).aggregate(Max('number')) is the path, but how do I ensure there is no race conditions between Max() and F()?

Dupuy

Which database are you using? – Flavorful

@Flavorful Postgres 9.4 – Dupuy

@MTaqi an auto increment field will increment regardless of which user the Invoice belongs to. I need to be able to have Invoice.number=1 for all users. Thinks of your own telco invoice numbering: it starts from 1 and increments until you call to cancel the service, for each client. – Dupuy

It'd be nice to have something like models.PositiveIntegerField(auto_increment_for('owner')) – Dupuy

Why not have a date field, which will tell you the latest invoice as well for each user. – Beulahbeuthel

@LegoStormtroopr Because I need invoices to be numbered as a sequence. That seams to be the most natural way. – Dupuy

Answer 1

You can achieve this and similar functions by overriding save method in model and writing your custom logics to it.

class Invoice(models.Model):
    owner  = models.ForeignKey(settings.AUTH_USER_MODEL)
    data   = models.TextField(default=None, blank=True, null=True)
    number = models.PositiveIntegerField(default=0, null=False)

    def save(self, *args, **kwargs):
        if self.pk:
            self.number += 1
        # Write all your logic here, like handeling max value etc
        return super(Invoice, self).save(*args, **kwargs)

Surrebutter

Ok, but that won't prevent a race condition, when 2 concurrent clients try to save at (almost) the same time. That is why I believe it must be done using F() expression. That way the increment operation is done at the database level. – Dupuy

you can handle IntegrityError and wait for random time to avoid race condition. ref: https://mcmap.net/q/606706/-handling-race-condition-in-model-save – Surrebutter

May be F() is better option. I haven't used it ever. – Surrebutter

Here is the best solution to your problem: check latest saved invoice: https://mcmap.net/q/1169454/-auto-increament-the-invoice-number-in-django-backend-for-new-invoice – Surrebutter

This is not a solution for multithreading applications, neither are any of both options in the comments. – Financial

Answer 2

you can get your first or last object like this:

# For last Object
Model.objects.latest('field') # Field can be id or pk or ...

# For first Object
Model.objects.all().first() # You can also use it on filter

Tetrapterous

Answer 3

A simple solution is you can make the number field as the primary key since its nature would be similar.

class Invoice(models.Model):
    owner  = models.ForeignKey(settings.AUTH_USER_MODEL)
    data   = models.TextField(default=None, blank=True, null=True)
    number = models.IntegerField(primary_key=True)

Or, you can make number as AutoField or BigAutoField.

number = models.AutoField()

Paapanen

Answer 4

use select_for_update() inside a transaction like this

with transaction.atomic():
    latest_invoice = Invoice.objects.filter(owner=user).select_for_update().aggregate(Max('number'))
    max_number = latest_invoice['number__max']
    next_number = (max_number or 0) + 1
    invoice = Invoice.objects.create(owner=user, data=data, number=next_number)

can find relevant documnetation here:- https://docs.djangoproject.com/en/5.1/ref/models/querysets/#select-for-update

Parotid