Inheritance

Inheritance and discriminators

model B extends A becomes allOf: [$ref to A, B's own properties]. @discriminator adds AsyncAPI 3.x's string-form discriminator naming the property's wire name:

@discriminator("kind")
model OrderEvent {
  kind: string;
  occurredAt: utcDateTime;
}

model OrderCreated extends OrderEvent {
  kind: "order-created";
  orderId: string;
}

model OrderCanceled extends OrderEvent {
  kind: "order-canceled";
  reason?: string;
}

components:
  schemas:
    OrderEvent:
      type: object
      properties:
        kind:
          type: string
        occurredAt:
          type: string
          format: date-time
      required:
        - kind
        - occurredAt
      discriminator: kind
    OrderCreated:
      allOf:
        - $ref: "#/components/schemas/OrderEvent"
        - type: object
          properties:
            kind:
              type: string
              enum:
                - order-created
            orderId:
              type: string
          required:
            - kind
            - orderId
    OrderCanceled:
      allOf:
        - $ref: "#/components/schemas/OrderEvent"
        - type: object
          properties:
            kind:
              type: string
              enum:
                - order-canceled
            reason:
              type: string
          required:
            - kind

Two rules the emitter enforces (each with a diagnostic when violated): the discriminating property must be defined on the model (or an ancestor), and it must be required. Otherwise discriminator is omitted with a warning rather than emitted broken.