Patterns, gotchas and small wins from fifteen-odd years of .NET and web work — the Regira packages and modules, entity pipelines, editorial front-ends and everything between.
One customer must never see another's rows, in any list, count or endpoint, including the one added next year. Seven requirements that make that a property of the system, and which of them no library can meet for you.
Rules that must hold for every record, such as hiding deleted data or showing each customer only their own, set up once and applied everywhere. They also cover the endpoint added next year, and nobody skips them by accident.