TransactionTestCase will truncate data created from data migration

[Jared Mackey]

Data migrations that create data in the database do not persist after running tests that inherit from TransactionTestCase. The first test that runs on the database works as expected but subsequent tests do not.

Here is an example migration:

def add_group_permissions(apps, schema_editor):
    for app_config in apps.get_app_configs():
        app_config.models_module = True
        create_permissions(app_config, verbosity=0)
        app_config.models_module = None

    Group = apps.get_model('auth', 'Group')
    Permission = apps.get_model('auth', 'Permission')

    group, _ = Group.objects.get_or_create(name='demo_group')
    group.permissions.clear()
    permission = Permission.objects.get(codename='existing_permission')
    group.permissions.add(permission)


class Migration(migrations.Migration):

    dependencies = [ ]

    operations = [
        migrations.RunPython(add_group_permissions, reverse_code=migrations.RunPython.noop),
        migrations.RunPython(lambda apps, schema_editor: ContentType.objects.clear_cache()),   # hack to get the ContentType cache to clear
    ]

Here is a test case that will prove that the second test to run will fail with django.contrib.auth.models.DoesNotExist: Group matching query does not exist.

class TestGroupsExist(TransactionTestCase):
    """ One of these tests will fail. Which one depends on which one the test runner runs first. The second one to run will fail. """ 
    def test_group_exists_1(self):
         self.assertIsNotNone(Group.objects.get_by_natural_key('demo_group'))

    def test_group_exists_2(self):
         self.assertIsNotNone(Group.objects.get_by_natural_key('demo_group'))

The following log is found after the first test is ran.

TRUNCATE ... "auth_group_permissions", "auth_group", ...;; args=None

A possible solution to this is to re-run data only migrations after truncating the tables.