Add test case for db access outside main thread in sync context

Author: Lode Rosseel

tests/test_db_thread_safeguards.py

@@ -0,0 +1,36 @@
+from __future__ import annotations
+
+import threading
+
+import pytest
+
+from pytest_django_test.app.models import Item
+
+
+@pytest.mark.django_db
+def test_sync_db_access_in_non_main_thread_is_blocked() -> None:
+    """
+    Ensure that when using the sync django_db helper (non-transactional),
+    database access from a different thread raises the expected RuntimeError
+    stating that DB access is only allowed in the main thread.
+
+    Mirrors the intent of the async equivalent test that checks thread
+    safeguards for async contexts.
+    """
+    captured: list[BaseException | None] = [None]
+
+    def worker() -> None:
+        try:
+            # Any ORM operation that touches the DB will attempt to ensure a connection.
+            # This should raise from the "sync_only" db blocker in non-main threads.
+            Item.objects.count()
+        except BaseException as exc:  # noqa: BLE001 - we want to capture exactly what is raised
+            captured[0] = exc
+
+    t = threading.Thread(target=worker)
+    t.start()
+    t.join()
+
+    assert captured[0] is not None, "Expected DB access in worker thread to raise an exception"
+    assert isinstance(captured[0], RuntimeError)
+    assert "only allowed in the main thread" in str(captured[0])