CODEWITHSUNDEEP
pythondjangodjango-models
Toufiqur Rahman
Toufiqur Rahman

Reputation: 212

Multiply two fields in django

I have two different models

class Part(models.Model):
    partno = models.CharField(max_length=50)   
    price = models.PositiveIntegerField(default= 0)
    tax = models.PositiveIntegerField(default= 0)
    created_date = models.DateField(auto_now_add=True)

class PurchaseOrder(models.Model):

    part = models.ForeignKey(Part, on_delete=models.CASCADE)
    po_quantity = models.PositiveIntegerField(default= 0)
    amount = models.PositiveIntegerField(default= 0)
    supplier = models.ForeignKey(Supplier, on_delete=models.CASCADE)
    product = models.ForeignKey(Product, on_delete=models.CASCADE)
    created_date = models.DateField(auto_now_add=True)

I'd like to multiply price from Part with po_quantity and result show in the amount. e.g

amount = price * po_quantity

Can someone assist me to solve this?

Upvotes: 1

Views: 352

Answers (1)

willeM_ Van Onsem
willeM_ Van Onsem

Reputation: 476554

Since amount fully depends on the price and the po_quantity, you better use a property for this. Otherwise you introduce data duplication. You thus can implement this as:

class PurchaseOrder(models.Model):
    part = models.ForeignKey(Part, on_delete=models.CASCADE)
    po_quantity = models.PositiveIntegerField(default= 0)
    supplier = models.ForeignKey(Supplier, on_delete=models.CASCADE)
    product = models.ForeignKey(Product, on_delete=models.CASCADE)
    created_date = models.DateField(auto_now_add=True)

    @property
    def amount(self):
        return self.po_quantity * self.part.price

If you need to use the amount when filtering, you can use .annotate(…) [Django-doc] instead, and define this for example in a Manager:

from django.db.models import F

class PurchaseOrderManager(models.Manager):
    
    def get_queryset(self, *args, **kwargs):
        return super().get_queryset(*args, **kwargs).annotate(
            amount=F('po_quantity') * F('part__price')
        )


class PurchaseOrder(models.Model):
    part = models.ForeignKey(Part, on_delete=models.CASCADE)
    po_quantity = models.PositiveIntegerField(default= 0)
    supplier = models.ForeignKey(Supplier, on_delete=models.CASCADE)
    product = models.ForeignKey(Product, on_delete=models.CASCADE)
    created_date = models.DateField(auto_now_add=True)

    objects = PurchaseOrderManager()

Upvotes: 1

Related Questions