If you've reached for acts_as_tenant or apartment without really understanding what's happening underneath, this tutorial is the correction. We're building a row-level multitenant system from first principles — the kind you can actually reason about when something goes wrong at 2am. By the end, you'll have: Middleware that resolves the current tenant from a subdomain or JWT claim A Current -based tenant context that threads safely through the request lifecycle An ApplicationRecord mixin that enforces tenant scoping at the model layer Postgres row-level security policies as a hard backstop A TenantSafe concern for background jobs that re-establishes context correctly A test helper that won't let you accidentally write cross-tenant specs No gems. Just Rails, Postgres, and honest code. The Mental Model Row-level multitenancy means every tenant's data lives in the same tables , separated by a tenant_id foreign key. The application layer is responsible for filtering.…