How could we express a model with a base model and with additional properties?

[ArcturusZhang]

In our current practice, additional properties on a model is expressed as

model Foo extends Record<unknown> {}

or

model Foo is Record<unknown> {}

where the Record<> takes the position of base model.

What if the model Foo needs a base model?

We have a similar case in swagger: https://github.com/Azure/azure-rest-api-specs/blob/64260c287d69860340a7c932f1450933dbb2c886/specification/datafactory/resource-manager/Microsoft.DataFactory/stable/2018-06-01/datafactory.json#L6381
This might be an issue when we try to migrate this from swagger to typespec.

And as a superset of swagger, I believe typespec should have the ability to define a model with both base model and additional properties.

BTW this will compile:

model Foo {}

model Bar extends Foo {
  name: string;

  ...Record<unknown>
}

The corresponding openAPI2.0 result from this tsp looks like this:

  "definitions": {
    "Bar": {
      "type": "object",
      "properties": {
        "name": {
          "type": "string"
        }
      },
      "required": [
        "name"
      ],
      "allOf": [
        {
          "$ref": "#/definitions/Foo"
        }
      ]
    },
    "Foo": {
      "type": "object"
    }
  },

If I remove the line ...Record<unknown>, it does not change.

The result does not really have anything for the spreaded Record, which might be a bug: we should either support this fully or forbid it instead of ignoring it.
And if making a decorator is not an option, I believe making the above tsp working as expected might be the solution of this issue to enable a model to both have base model and additional properties.

[timotheeguerin]

Design issue here microsoft/typespec#2442