Why does IDocumentRtService throw an exception if a computed field is not present in the document?

Kernel
1

While working with computations I came across a surprising behaviour of IDocumentRtService.

For example if there is this document and a corresponding model:

{
    "id": "123",
    "Root": {
        "Foo": null
    }
}

The model contains a computation for field Foo. Then using iDocumentRtService.compute(document, language) succeeds. But if I use the following document the computation fails:

{
    "id": "123",
    "Root": {
    }
}

In the latter document the field is not present and the call to iDocumentRtService.compute(document, language) fails with:

Caused by: com.mgmtp.a12.md.rt.api.ComputationException: Field instance '/Root[1]/Foo[1]' not found!
	at com.mgmtp.a12.md.internal.rt.ComputationResultImpl.createComputedValue(ComputationResultImpl.java:79)
	at com.mgmtp.a12.md.internal.rt.ComputationResultImpl.<init>(ComputationResultImpl.java:57)
	at com.mgmtp.a12.md.internal.rt.DocumentDynamicRtService.compute(DocumentDynamicRtService.java:96)
	... 1 more

Is this intended behaviour? If the field is computed anyway shouldn’t it then be created automatically?

A workaround for this can be found here: How to add missing fields to a document?