In django how to model an invoice which can have multiple items? - django

Question

I am using Django Rest Framework in the backend and Angular for frontend.
If the customer order multiple items, it should be be in a single invoice. For example, if the customer order apple, orange and banana, all these should be in a single invoice.
When the customer order again and it will be a new invoice.
class InvoiceItem(models.Model):
product = models.ForeignKey(
Product, on_delete=models.CASCADE, related_name='invoiceitems')
customer = models.ForeignKey(
Customer, on_delete=models.CASCADE, related_name='invoiceitems')
quantity = models.PositiveIntegerField()
date = models.DateTimeField(auto_now_add=True)
class Invoice(models.Model):
invoice_item = models.OneToOneField(
InvoiceItem, on_delete=models.CASCADE, related_name='invoice')
Now I have to link the InvoiceItem with Invoice. I thought about using post_save signals with InvoiceItem as a sender to create Invoice object and link it with the InvoiceItem.
#receiver(signals.post_save, sender=InvoiceItem)
def create_account(sender, instance, created, **kwargs):
Invoice.objects.update_or_create(invoice_item=instance)
How can I do it for multiple items?
Or there is a better way to implement my requirements?

Answer 1

There are multiple problems with your approach.
First of all, your
invoice_item = models.OneToOneField(
InvoiceItem, on_delete=models.CASCADE, related_name='invoice')
implies that there is only one type of Product per Invoice, i.e. having bought apples Customer can't buy also some oranges in the same Invoice.
If you try to fix that by creating ForeignKey on InvoiceItem, as others have already pointed out, you find yourself struggling with the second problem:
You attach InvoiceItem to a customer. Meaning that there's nothing stopping the system from creating a single Invoice that has 5 Oranges bought by Alice as an InvoiceItem and 7 Apples bought by Bob as another InvoiceItem, which seems wrong.
I would also move the date field from InvoiceItem to Invoice and rename it to timestamp, as I assume an Invoice is a set of Products bought together at one time.
You would end up with something like this:
class InvoiceItem(models.Model):
invoice = models.ForeignKey(
Invoice, on_delete=models.CASCADE, related_name='invoice_items')
product = models.ForeignKey(
Product, on_delete=models.CASCADE, related_name='invoice_items')
quantity = models.PositiveIntegerField()
class Invoice(models.Model):
customer = models.ForeignKey(
Customer, on_delete=models.CASCADE, related_name='invoices')
timestamp = models.DateTimeField(auto_now_add=True)
Now if you examine related_name arguments on the fields you'll see that you can do very useful things with your objects, like
customer = Customer.objects.get(id=<some_id>)
customer.invoices
will give you a QuerySet of all the Invoices belonging to a specific Customer. And having an Invoice object you can
invoice.invoice_items
to get all the items on an invoice
Or, for example, having a Product, say, 'apple', you can find all customers that ever bought an apple:
# Find all invoice_items for apple
inv_items = apple_product.invoice_items.all()
# Filter Customers
Customer.objects.filter(invoices__invoice_items__in=inv_items)
If you squint a little, you'll see that this whole structure is just a django M2M through relation (https://docs.djangoproject.com/en/3.0/topics/db/models/#extra-fields-on-many-to-many-relationships) and can be rewritten like this:
class InvoiceItem(models.Model):
invoice = models.ForeignKey(
Invoice, on_delete=models.CASCADE, related_name='invoice_items')
product = models.ForeignKey(
Product, on_delete=models.CASCADE, related_name='invoice_items')
quantity = models.PositiveIntegerField()
class Invoice(models.Model):
customer = models.ForeignKey(
Customer, on_delete=models.CASCADE, related_name='invoices')
timestamp = models.DateTimeField(auto_now_add=True)
products = models.ManyToManyField(Product, through='InvoiceItem')
So now you have Invoice directly connected to Products, and this connection also contains information about the quantity of this Product on this Invoice. I will leave the exploration of the benefits of this approach to the reader.

Answer 2

You should use a ForeignKey from the InvoiceItem to Invoice:
class InvoiceItem(models.Model):
...
invoice = models.ForeignKey(
Invoice, on_delete=models.CASCADE
)
...
You should be able to then remove the post_save. You'll create items like this:
>>> invoice = Invoice.objects.create(<insert arguments here>)
>>> invoice_item = InvoiceItem.objects.create(invoice=invoice, <insert other arguments>)
>>> invoice_item.invoice.pk == invoice.pk
[out] True (i.e., the invoice item is attached to the invoice)