[MPG] Empty models in tsp, what should we generate

[XiaofeiCao]

tsp: https://github.com/Azure/azure-rest-api-specs/blob/main/specification/devopsinfrastructure/Microsoft.DevOpsInfrastructure/main.tsp#L314-L315

converted swagger: https://github.com/Azure/azure-rest-api-specs/blob/main/specification/devopsinfrastructure/resource-manager/Microsoft.DevOpsInfrastructure/preview/2024-04-04-preview/devopsinfrastructure.json#L1511-L1514

TCGC issue: Azure/typespec-azure#846
copied from the issue:

Swagger	m4	TypeSpec	TCGC
{"type": "object"}	anyObject	{}	anonymous model with no properties
{"type": "object"}	anyObject	model EmptyModel {}	named model with no properties
{}	any	unknown	any

Considerations:
We probably need to check what code-model would it be, if it is

1. {} from tsp
2. model Model {} from tsp
   Is it only anonymous model vs named model? Do we want to generate them both as empty class (DPG would do this)?

And can we distinguish this "empty model" from typespec, from e.g. "type": "object" from Swagger?

[XiaofeiCao]

Is it only anonymous model vs named model?

From code-model point of view, it is. The generated code-model is the same. Though TCGC's result contains isGeneratedName for SdkModelType:

And can we distinguish this "empty model" from typespec, from e.g. "type": "object" from Swagger?

I'm not sure if TCGC has a way to represent anyObject in M4. If it has, we can modify our old M4 logic to detect anyObject(instead of treating it as AnySchema). If not, guess we can't.

Likely they will use {} to represent anyObject: Azure/typespec-azure#846 (comment). This case, we probably want to generate empty model for named model, but not for anonymous model {}. Need to confirm with TCGC.

[XiaofeiCao]

Discussion here: Azure/typespec-azure#846 (comment)

1. Empty model in TypeSpec means model without properties, not anyObject.
2. For brownfields, by default we convert anyObject as TypeSpec (anonymous)empty model. If service really wants anyObject, TypeSpec's equivalence is Record<unknown>.

@weidongxu-microsoft Guess for TypeSpec greenfield in MPG, we treat anonymous/named empty model as empty class. While for brownfields(since by default they convert anyObject to anonymous empty model), we treat them as Object to avoid breaking change?

[weidongxu-microsoft]

I think for tsp, empty model generate empty class.

Compatibility with brownfield is another topic. And we may not need exact same result (one reason is that m4 don't return empty model in the first place, and we likely won't solve that).

[XiaofeiCao]

Makes sense.
Guess we first extend ObjectMapper in TypeSpec:

autorest.java/javagen/src/main/java/com/azure/autorest/mapper/ObjectMapper.java

Line 86 in b8d4f4d

return !JavaSettings.getInstance().isDataPlaneClient()

And do a case by case for brownfields in the future.