[Django] #36043: TransactionTestCase erasing migration data when using --keepdb option

[Django]

#36043: TransactionTestCase erasing migration data when using --keepdb option
-------------------------------------+-------------------------------------
Reporter: Guillaume LEBRETON | Type:
| Uncategorized
Status: new | Component:
| Documentation
Version: 5.1 | Severity: Normal
Keywords: | Triage Stage:
| Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
I've made a project for this issue to be easily reproduced here:
https://github.com/Guilouf/django_bug
Using django5.1.4 and postgresql-17

When object are created in a data migration, and when there is a
`TransactionTestCase` among the test case, all data is erased at the end
of the tests run.
Then, if you try to re-run the tests using the `--keepdb` option, the data
created during migrations is gone.

Here is how to reproduce it:

models.py
{{{
from django.db import models

class Car(models.Model):
name = models.CharField(max_length=100)
}}}

data migration

{{{
from django.db import migrations, models


def create_car(apps, schema_editor):
Car = apps.get_model("cars", "Car")
Car.objects.create(name="206")

class Migration(migrations.Migration):
dependencies = [
('cars', '0001_initial'),
]

operations = [migrations.RunPython(create_car,
migrations.RunPython.noop) ]

}}}

keepdb test
{{{
from django.test import TestCase, TransactionTestCase

from cars.models import Car


class A_CarTestCase(TestCase):

def test_car_migrated(self):
self.assertTrue(Car.objects.first())


class B_BugTestCase(TransactionTestCase):

def test_bug_migrated(self):
pass
}}}

Using ` python manage.py test cars.tests.tests_keepdb --keepdb`, the first
time tests are executed correctly, the second time `test_car_migrated`
fails because there is no more existing cars.

I tried using `serialized_rollback = True` but it doesn't change anything
about keeping data after the test session ends. I may have found some
issues with that option but it should be on a separate ticket.

I understand that is not necessarily a bug, but this implications should
appear in the docs that you can't mix `--keepdb` and having just even one
`TransactionTestCase` if you want to access data from migrations.
https://docs.djangoproject.com/en/5.1/topics/testing/overview/#preserving-
the-test-database I think the section of the doc should be updated with a
warning about `TransactionTestCase`.

Here,
https://docs.djangoproject.com/en/5.1/topics/testing/tools/#transactiontestcase,
we can read "''A TransactionTestCase may call commit and rollback and
observe the effects of these calls on the database.''", i think this is
misleading and should be reformulated, when i read this i think that the
database would be restored to the state it was before the test, which is
not the case.

Here https://github.com/wagtail/wagtail/issues/4520 an example of
implications of this behavior. Wagtail have some models (`Page` for
example) created during migrations. If you use even just one
`TransactionTestCase`, then after a test re-run with `--keepdb` all the
mandatory data from wagtails migrations are gone.

I can propose a PR to update the docs if you agree.
--
Ticket URL: <https://code.djangoproject.com/ticket/36043>
Django <https://code.djangoproject.com/>
The Web framework for perfectionists with deadlines.

[Django]

#36043: TransactionTestCase erasing migration data when using --keepdb option

------------------------------------+--------------------------------------
Reporter: Guillaume LEBRETON | Owner: (none)
Type: Bug | Status: closed
Component: Testing framework | Version: 5.1
Severity: Normal | Resolution: duplicate

Keywords: | Triage Stage: Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0

------------------------------------+--------------------------------------
Changes (by Tim Graham):

* component: Documentation => Testing framework
* resolution: => duplicate
* status: new => closed
* type: Uncategorized => Bug

Comment:

Duplicate of #25251.
--
Ticket URL: <https://code.djangoproject.com/ticket/36043#comment:1>