Creating an ID that rolls over by year - django

Question

I have an invoice app that has an ID. I'd like to have it rollover by year. That is, every year, it would start back at 001. My return string would be 2014-001, 2014-002, 2015-001...
However, Django doesn't seem to support composite keys. Is there a way to do this?
class Invoice(models.Model):
description = models.CharField(max_length=200)
date = models.DateField('Date Issued')
client = models.ForeignKey('organizations.Contact')
UNPAID = 'Unpaid'
PAID = 'Paid'
STATUS_CHOICES = (
(UNPAID, 'Unpaid'),
(PAID, 'Paid'),
)
status = models.CharField(max_length=6,
choices=STATUS_CHOICES, default=UNPAID)
notes = models.TextField(blank=True)
def __str__(self):
return (self.date.__str__().replace("-", "") +
"-" + self.id.__str__().zfill(3))
edit new code:
def save(self, *args, **kwargs):
if self.invoice_id is "":
current_date = self.date.year
previous_invoice = Invoice.objects.filter(invoice_id__contains=current_date).order_by('-id').first()
if previous_invoice is not None:
num = int(previous_invoice.invoice_id[5:])
else:
num = 1
self.invoice_id = '{year}-{num:03d}'.format(year=current_date, num=num)
super(Invoice, self).save(*args, **kwargs)

Answer 1

You can use a CharField for the id, setting it as primary key and generating it automatically upon saving.
Something similar to the following should work:
class Invoice(models.Model):
id = models.CharField(max_length=8, null=False, primary_key=True)
...
def save(self, *args, **kwargs):
if self.pk is None:
current_year = datetime.now().year
previous_invoice = Invoice.objects.filter(id__contains=str(year)).order_by('-id').first()
if previous_invoice is not None:
num = int(previous_invoice.id[5:])
else:
num = 1
self.pk = '{year}-{num:03d}'.format(year=current_year, num=num)
super(Invoice, self).save(*args, **kwargs)
EDIT: Add missing primary_key attribute in id field definition.