At a basic level - database migrations must be backwards compatible at least with the previous version, and they go out before the service update is deployed.

At a basic level - database migrations must be backwards compatible at least with the previous version, and they go out before the service update is deployed. Unit tests don't run direct against prod usually, but regardless they would be run after migrations to a database of (production schema+migrations). Each environment - dev, test, staging, prod has its own db. Even spinning up an ephemeral db per test is possible, and easy with containers. YMMV as system complexity increases, but by then there should be whole team(s) managing the issue