Sumproduct using Django's aggregation
- by Matthew Rankin
Question
Is it possible using Django's aggregation capabilities to calculate a sumproduct?
Background
I am modeling an invoice, which can contain multiple items. The many-to-many relationship between the Invoice and Item models is handled through the InvoiceItem intermediary table.
The total amount of the invoice—amount_invoiced—is calculated by summing the product of unit_price and quantity for each item on a given invoice. Below is the code that I'm currently using to accomplish this, but I was wondering if there is a better way to handle this using Django's aggregation capabilities.
Current Code
class Item(models.Model):
item_num = models.SlugField(unique=True)
description = models.CharField(blank=True, max_length=100)
class InvoiceItem(models.Model):
item = models.ForeignKey(Item)
invoice = models.ForeignKey('Invoice')
unit_price = models.DecimalField(max_digits=10, decimal_places=2)
quantity = models.DecimalField(max_digits=10, decimal_places=4)
class Invoice(models.Model):
invoice_num = models.SlugField(max_length=25)
invoice_items = models.ManyToManyField(Item,through='InvoiceItem')
def _get_amount_invoiced(self):
invoice_items = self.invoiceitem_set.all()
amount_invoiced = 0
for invoice_item in invoice_items:
amount_invoiced += (invoice_item.unit_price *
invoice_item.quantity)
return amount_invoiced
amount_invoiced = property(_get_amount_invoiced)
