serialized_rollback fixture is not available in TransactionTestCase.setUpClass

[Jacob Walls]

serialized_rollback=True allows subsequent TransactionTestCase classes to continue depending on the data created in the app's initial migration after a previous TransactionTestCase truncated tables.

By trial and error, I found that I couldn't depend on this emulated rollback during the setUpClass() phase, but rather during the setUp() phase only. This results in a difficult DX when debugging setUpClass() failures in tests that pass in isolation, and resorting to setUp() (as I ultimately did) has a performance penalty.

TestCase loads fixtures before running setUpClass(). Can we explore letting TransactionTestCase do the same? I imagine the refactor involves implementing TransactionTestCase.setUpClass().

Nothing in the docs advises that the emulated rollback is only available during certain parts of the test lifecycle, so at the very least I'd suggest a docs update. Happy to cross-post to forum if this isn't a "bug".

---

Sample project

models.py

from django.db import models

class MyModel(models.Model):
    flag = models.BooleanField()

migrations/0001_initial.py

# Generated by Django 4.2.15 on 2024-08-06 19:34

from django.db import migrations, models

def seed_data(apps, schema_editor):
    MyModel = apps.get_model("fooapp", "MyModel")
    MyModel.objects.create(flag=True)


class Migration(migrations.Migration):

    initial = True

    dependencies = []

    operations = [
        migrations.CreateModel(
            name="MyModel",
            fields=[
                (
                    "id",
                    models.BigAutoField(
                        auto_created=True,
                        primary_key=True,
                        serialize=False,
                        verbose_name="ID",
                    ),
                ),
                ("flag", models.BooleanField()),
            ],
        ),
        migrations.RunPython(seed_data, migrations.RunPython.noop),
    ]

tests.py

from django.test import TransactionTestCase

from .models import MyModel

def depend_on_fixture():
    assert MyModel.objects.count()


class A(TransactionTestCase):
    serialized_rollback = True

    @classmethod
    def setUpClass(cls):
        depend_on_fixture()

    def test_a(self):
        pass


class B(TransactionTestCase):
    serialized_rollback = True

    @classmethod
    def setUpClass(cls):
        depend_on_fixture()

    def test_b(self):
        pass

Gives only one failure, which you'll miss if developing the tests in isolation:

======================================================================
ERROR: setUpClass (fooapp.tests.B)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/jwalls/foo/fooapp/tests.py", line 25, in setUpClass
    depend_on_fixture()
  File "/Users/jwalls/foo/fooapp/tests.py", line 6, in depend_on_fixture
    assert MyModel.objects.count()
AssertionError

----------------------------------------------------------------------
Ran 1 test in 0.004s

FAILED (errors=1)

[Sarah Boyce]

Replicated on main thank you for the detailed report
We could class this as a new feature to add support for this but I do agree that it isn't intuitive and so will leave it as a bug

[Jacob Walls]

​PR

[Sarah Boyce <42296566+sarahboyce@…>]

In 8eca3e9b:

Refs #35660 -- Updated TransactionTestCase methods into class or static methods.

[Sarah Boyce <42296566+sarahboyce@…>]

In a060a22e:

Fixed #35660 -- Made serialized_rollback and fixture data available in TransactionTestCase.setUpClass().

[Raphael Gaschignard]

#35921 is an issue that appeared with the fix for this ticket (unsure if this triggers a change in this ticket, or what flow is involved when an issue is found after a ticket is fixed)