Don't run _pre_setup() twice for transactional tests (#1293)

* Add test for repeated _pre_setup on transactional tests

Records who calls _pre_setup(). A plain django_db test is set up once, by us. A
django_db(transaction=True) test is set up twice: TransactionTestCase.setUpClass()
runs _pre_setup() eagerly and we then run it again.

* Don't run _pre_setup() twice for transactional tests

TransactionTestCase.setUpClass() runs _pre_setup() eagerly and flags it in
_pre_setup_ran_eagerly, which unittest.TestCase.run() clears instead of setting
the test up again. We drive the test case ourselves, so do the same.

Repeating _pre_setup() redid the whole fixture setup - serialized rollback
restore, fixtures, sequence reset - on every transactional test. With
available_apps it also sent setting_changed(enter=True) and post_migrate twice
against the single exit in _post_teardown.

Author: vokracko

docs/changelog.rst

@@ -14,6 +14,7 @@ Bugfixes
 
 * Fixed type hints of assert methods to match actual signature (`PR #1271 <https://github.com/pytest-dev/pytest-django/pull/1271>`__)
 * Handled Django 6.2's ``ImproperlyConfigured`` (in addition to ``ImportError``) when the configured ``DJANGO_SETTINGS_MODULE`` cannot be imported, so pytest-django still shows its guidance message.
+* Fixed ``django_db(transaction=True)`` tests being set up twice, which repeated the ``serialized_rollback`` restore and the ``fixtures`` load, and sent ``setting_changed`` and ``post_migrate`` twice when ``available_apps`` is set.
 
 v4.12.0 (2026-02-14)
 --------------------

pytest_django/fixtures.py

@@ -289,7 +289,10 @@ def tearDownClass(cls) -> None:
         PytestDjangoTestCase.setUpClass()
 
         test_case = PytestDjangoTestCase(methodName="__init__")
-        test_case._pre_setup()
+        if not PytestDjangoTestCase._pre_setup_ran_eagerly:
+            # For a TransactionTestCase, setUpClass() has already run _pre_setup() and set
+            # this flag to say so.
+            test_case._pre_setup()
 
         yield
 

tests/test_database.py

@@ -463,6 +463,46 @@ def test_db_access(self):
     result.assert_outcomes(passed=1)
 
 
+def test_pre_setup_runs_once_per_test(django_pytester: DjangoPytester) -> None:
+    """Each test is set up by exactly one _pre_setup() call.
+
+    A plain test is set up by us, a transactional one by Django's
+    TransactionTestCase.setUpClass(), which runs _pre_setup() eagerly.
+    """
+
+    django_pytester.create_test_module(
+        """
+        import sys
+
+        import pytest
+        from django.test import TransactionTestCase
+
+        # Name of the function that called _pre_setup(), appended once per call and
+        # never reset, so the list below is the setup history of the whole module.
+        callers = []
+        original_pre_setup = TransactionTestCase._pre_setup.__func__
+
+        @classmethod
+        def recording_pre_setup(cls):
+            callers.append(sys._getframe(1).f_code.co_name)
+            original_pre_setup(cls)
+
+        TransactionTestCase._pre_setup = recording_pre_setup
+
+        @pytest.mark.django_db
+        def test_plain_db():
+            assert callers == ["_django_db_helper"]
+
+        @pytest.mark.django_db(transaction=True)
+        def test_transactional_db():
+            assert callers == ["_django_db_helper", "setUpClass"]
+        """
+    )
+
+    result = django_pytester.runpytest_subprocess("-v", "--reuse-db")
+    result.assert_outcomes(passed=2)
+
+
 class Test_database_blocking:
     def test_db_access_in_conftest(self, django_pytester: DjangoPytester) -> None:
         """Make sure database access in conftest module is prohibited."""