how do people normally deal with class variables in django?

Question

I can't see any provision for this in the django docs, so how do people go about doing this.

My specific case is this.

I have a shopping cart, each cart instance has an invoice number field, however the invoice number is only generated if the cart goes to a paid status, so not all shopping cart instances will have an invoice number. I want all invoice numbers to be sequential with no gaps between them, so the default pk isn't perfect in this case, so I want a class variable that acts as a counter for the invoice numbers, and is accessable by all instances.

Answer 1

The default primary key will already be a unique monotonic integer (even in SQLite if you don't delete any records), so you can just use that for it.

Answer 2

You can create a field in a model and state it as primary key or use your current primary key if you are migrating a legacy database primary_key=True.

Django ORM has no support for complex keys. It doesn't allow you to use a table without primary key.

If you need a specific logic for your invoice number generation (for example # of client + '-' + year number), you can create a field for this key, but this will not be a primary key. Call this key generation function on object save in Model.save function if this key is not specified for current object yet.

Answer 3

Have your invoice field in your model:

class Cart(models.Model):
 .....
 invoice_id = models.PositiveIntegerField(null=True,unique=True)

Only set the value when appropriate.

To find the next ID:

try:
  nextID = Cart.objects.exclude(invoice_id=None).order_by('-invoice_id')[0].invoice_id + 1
except IndexError:
  nextID = 1

Having thought about it some more... it occurs to me that invoices might be better served as a separate model. You can link them to your carts with ForeignKey("invoice",null=True). They can then be created when needed, the default primary key field can be your monotonically increasing number, and can then have a separate life-cycle to your cart objects.