Document that transactions do not revert model state

[Todor Velichkov]

I know this may be obvious, but I think its worth mentioning in the documentation that when a transaction rollback happen your model/s state will not be reverted, thus the underlying model instance/s may end up in an inconsistent state if not manually reverted.

A simple toy example:

from django.db import IntegrityError, transaction
from django.contrib.auth.models import User

def update_user(pk, username):
    user = User.objects.get(pk=pk)
    try:
        with transaction.atomic():
            user.username = username
            user.is_active = False
            user.save()
            return user
    except IntegrityError:
        return user

since username is an unique field if an IntegrityError happen and the transaction got reverted, the user which we return will have an inconsistent state (if not manually reverted).

[Tim Graham]

The document to update is probably docs/topics/db/transactions.txt.

[Jonatas CD]

Here is the ​PR.

[Tim Graham <timograham@…>]

In c9b22707:

Fixed #28479 -- Doc'd that transaction rollback doesn't revert model state.

[Tim Graham <timograham@…>]

In 0221039:

[1.11.x] Fixed #28479 -- Doc'd that transaction rollback doesn't revert model state.

Backport of c9b22707b0703db6c6ddaebdd00e2cd33d182e40 from master