workx_backend/invoice/models.py

from django.db import models
from project.models import Project


class Invoice(models.Model):
    title = models.CharField(max_length=30)
    project = models.ForeignKey(Project, on_delete=models.CASCADE)
    invoice_id = models.CharField(max_length=15)
    raised_date = models.DateField()
    due_date = models.DateField()
    paid_date = models.DateField(null=True, blank=True)
    is_archived = models.BooleanField(default=False)

    @property
    def invoice_total(self):
        items = Item.objects.filter(invoice=self.pk)
        total = 0
        for item in items:
            total += item.item_total
        return total

    def __str__(self):
        return self.invoice_id


class Item(models.Model):
    name = models.TextField(blank=False, null=False)
    amount = models.DecimalField(max_digits=15, decimal_places=2)
    quantity = models.IntegerField()
    invoice = models.ForeignKey(Invoice, on_delete=models.CASCADE)

    @property
    def item_total(self):
        return self.amount * self.quantity

    def __str__(self):
        return self.name