Concurrency fix 2: pessimistic locking with select_for_update()

# ======================================================================
# Q: How do you use pessimistic locking with select_for_update() to prevent race
# conditions?
# ======================================================================

from django.db import transaction

with transaction.atomic():
    account = Account.objects.select_for_update().get(pk=account_id)  # SELECT ... FOR UPDATE
    if account.balance < amount:
        raise InsufficientFunds()
    account.balance -= amount
    account.save(update_fields=['balance'])
    Ledger.objects.create(account=account, amount=-amount)
# the lock is released at COMMIT