class Worker(models.Model):
    name = models.CharField(max_length=255)


class Invoice(models.Model):
    worker = models.ForeignKey(
        'Worker', on_delete=models.CASCADE)
    amount = models.DecimalField(max_digits=10, decimal_places=2)

from decimal import Decimal

from django.test import TestCase
from django.db.models import Sum

from myapp.models import Worker, Invoice


def get_payable_invoices():
    return Invoice.objects.filter(
        worker__in=Worker.objects.annotate(Sum('invoice__amount')))
        .filter(invoice__amount__sum__gt=0)


class PayableInvoicesTests(TestCase):
    def test_get_payable_invoices(self):
        worker1 = Worker.objects.create(name="John Doe")

        invoice1 = Invoice.objects.create(
            worker=worker1, amount=Decimal('100.00'))
        invoice2 = Invoice.objects.create(
            worker=worker1, amount=Decimal('-150.00'))

        worker2 = Worker.objects.create(name="Mary Contrary")
        invoice3 = Invoice.objects.create(
            worker=worker2, amount=Decimal('200.00'))

        self.assertEqual(get_payable_invoices().count(), 1)
        self.assertEqual(get_payable_invoices().first(), invoice3)

django.core.exceptions.FieldError: Cannot resolve keyword 'invoice' into field. Choices are: amount, id, worker, worker_id