Why the Repository Pattern in NestJS Deserves a Second Look

NestJS applications backed by TypeORM commonly inject repositories directly into services, allowing developers to query and persist data with minimal setup. While this approach works reliably in production, it tightly couples business logic to persistence concerns such as ORM-specific query syntax, null-return behavior, and implicit save semantics. A key structural issue arises when the domain entity doubles as the database schema definition, blending business rules with persistence annotations in the same class. This design carries little visible cost on simple, single-table modules but creates measurable complexity as entities accumulate invariants, shared queries, and rules that must be testable without a database. The article argues that deliberately separating the business model from the persistence model becomes worthwhile once those three conditions emerge in a real-world domain.