composition-over-inheritance

Full name

@azure-tools/typespec-azure-core/composition-over-inheritance

Models that extend a base model without a discriminator should use composition (... spread or model is) instead of inheritance (extends). If polymorphism is intended, add the @discriminator decorator on the base model.

❌ Incorrect

Inheritance without a discriminator:

model Pet {
  name: string;
}

model Cat extends Pet {
  meow: boolean;
}

Extending a template instance:

model Pet<T> {
  t: T;
}

model Cat extends Pet<string> {}

✅ Correct

Using composition with spread:

model Pet {
  name: string;
}

model Cat {
  ...Pet;
  meow: boolean;
}

Using model is:

model Pet {
  name: string;
}

model Cat is Pet {
  meow: boolean;
}

Inheritance with a discriminator for polymorphism:

@discriminator("kind")
model Pet {
  kind: string;
  name: string;
}

model Cat extends Pet {
  kind: "cat";
  meow: boolean;
}

model Dog extends Pet {
  kind: "dog";
  bark: boolean;
}