Test transactional rollback on a service that delegates commit lifecycle to a framework session dependency

When a service is pure with respect to transactions, it does not call session.commit() or session.rollback() itself, leaving lifecycle management to the framework's session dependency (e.g. FastAPI's SessionDep that commits on clean return and rolls back on exception), unit tests that pin all-or-nothing behavior cannot rely on the dependency to roll back, because the dependency is not in the call stack. The test must (a) simulate a mid-batch failure (typically by monkeypatching session.flush to raise on the Nth call), (b) assert the exception propagates, (c) call session.rollback() explicitly, then (d) assert post-rollback state (e.g. zero rows in the affected scope). Step (c) is load-bearing for assertion correctness, not just hygiene: SQLAlchemy and SQLModel sessions enter an aborted state after a failed flush, and subsequent queries raise PendingRollbackError until rollback is called. Verified 2026-05 in a Python/FastAPI/SQLModel codebase against pytest 9 with module-scoped session fixtures.

For unit tests of services that delegate commit/rollback to a framework session dependency, simulate the failure (e.g. monkeypatch session.flush to raise on the Nth call), let it propagate out of the service, then call session.rollback() in the test itself before asserting post-rollback state. To choose N, count flush calls per item across the service's loop and pick a value that lands after at least one full item has flushed but before the next is complete. Verify the rollback call is required by removing it temporarily and observing PendingRollbackError on the assertion query.