a_models.py

class Account(models.Model):
    client =  models.ForeignKey(Client, related_name='client')
    transactions = models.ManyToManyField('Transaction')
    
    name = models.CharField(max_length=150, unique=True, null=True, blank=True)

    currency = models.ForeignKey('Currency', related_name='currency')
    is_usd_account = models.BooleanField(default=False)

    created = models.DateTimeField(auto_now_add=True)
    modified = models.DateTimeField(null=True, blank=True)
    
    class Meta:
        ordering = ['-created']
    
    def recalculate(self):
        pass
        # ...business logic:
        # analyzing all the transactions of account, updating 
        # some application-wide metrics and other stuff

    def display_name(self):
        return f"Account: {self.name}"
        
    def save(self, **kwargs):
        self.modified = datetime.now()
        super().save(**kwargs)


class Transaction(models.Model):
    account = models.ForeignKey(Account, on_delete=models.CASCADE)
    
    description = models.CharField(max_length=150, null=True, blank=True)
    amount = models.DecimalField(max_digits=12, decimal_places=4, null=True, blank=True)

    created = models.DateTimeField(auto_now_add=True)
    modified = models.DateTimeField(null=True, blank=True)
    
    def save(self, **kwargs):
        self.modified = datetime.now()
        self.account.transactions.add(self)
        self.account.recalculate()
        super().save(**kwargs)

b_utils.py

def get_client_statistics(client):
    """
    Return a total amount of all transactions of a client and the total number of transactions.
    :return tuple: (<total_amount>, <number>)
    """
    if not client:
        return None

    x = y = 0
    accounts = Account.objects.filter(client=client).all()

    for account in accounts:
        transactions = Transaction.objects.filter(account=account, amount__gt=0).all()
        y += transactions.count()

        for transaction in transactions:
            x += transaction.amount

    return x, y

c_0002_migration.py

from models import Account


def forwards(migration_apps, schema_editor):
    for account in Account.objects.all():
        account.name = account.display_name
        account.save()


class Migration(migrations.Migration):

    dependencies = [
        # ...
    ]

    operations = [
        migrations.RunPython(forwards, None),
    ]

d_test_stats.py

class StatisticsTestCase(TestCase):

    def setUp(self):
        self.client_a = Client.objects.create(...)
        self.account_a = Account.objects.create(client=self.client_a, ...)
        Transaction.objects.create(account=self.account_a, amount=2.5, ...)
        Transaction.objects.create(account=self.account_a, amount=-2, ...)
        Transaction.objects.create(account=self.account_a, amount=0.5, ...)

        self.client_b = Client.objects.create(...)
        self.account_b = Account.objects.create(client=self.client_b, ...)
        Transaction.objects.create(account=self.account_b, amount=-5, ...)
        Transaction.objects.create(account=self.account_b, amount=-2, ...)
        Transaction.objects.create(account=self.account_b, amount=0, ...)


    def test_statistics(self):
        self.assertIsNone(get_client_statistics(None))

        amount, number = get_client_statistics(self.client_a)
        self.assertEqual(amount, 3)
        self.assertEqual(number, 2)

        amount, number = get_client_statistics(self.client_b)
        self.assertEqual(amount, 0)
        self.assertEqual(number, 0)