domain

Domain Layer

The domain layer contains the pure business logic of a bounded context. It has zero framework dependencies except @nestjs/cqrs for AggregateRoot (entity base class). No @Injectable, no PrismaService, no HTTP concerns.

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Decision Tree

What are you creating?

Domain artifact needed?
│
├── An entity (aggregate root with lifecycle methods)
│   └── Go to: Entity Pattern
│
├── A simple value (email, amount, name, phone)
│   └── Go to: VO — Scalar
│
├── A grouped value (address, bank details, money)
│   └── Go to: VO — Composed
│
├── A fixed set of values (payment method, channel type)
│   └── Go to: VO — Enum
│
├── A status with allowed transitions (order status, withdrawal status)
│   └── Go to: VO — State Machine
│
├── The persistence contract for the entity
│   └── Go to: Repository Interface
│
├── Something that happened in the domain
│   └── Go to: Domain Events
│
├── Logic that belongs to the domain but not to one entity
│   └── Go to: Domain Services
│
├── A domain-specific error with context
│   └── Go to: Domain Errors
│
├── Entity invariant enforcement (field-level rules)
│   └── Go to: Validators
│
└── Fake data for tests
    └── Go to: Data Builders

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Entity Pattern

Full reference: references/entity-patterns.md

An entity is identified by its UniqueEntityID, not by its value. It extends AggregateRoot from @nestjs/cqrs so that events flow through the NestJS EventBus once committed.

Two factory methods are mandatory:

• static create(props, id?) — for new instances; calls this.apply(new XCreatedEvent(...))
• static restore(props, id) — for DB hydration; emits no events

Event lifecycle (every entity follows this):

entity.apply(event)            // 1. records the event in memory
publisher.mergeObjectContext(entity)  // 2. wires EventBus (in handler)
await repo.save(entity)        // 3. persists
entity.commit()                // 4. publishes events to EventBus

Condensed template: