Repeating primitive with extensions but no values omits the null-padded value array

[oliveregger]

When every entry of a repeating primitive element carries only an extension (or id) and no value, SUSHI emits the _element array without the parallel value array. Per the FHIR JSON spec the value array should still be present, null-filled, so the two arrays align.

SUSHI already gets this right as soon as one entry has a value — it emits "given": ["Xavier", null] alongside "_given": [null, {...}]. Only the all-values-absent case drops the value array, so this looks like a gap in an otherwise-correct code path rather than a design decision.

Version: SUSHI v3.20.0 (implements FHIR Shorthand specification v3.0.0), fhirVersion: 4.0.1.

Steps to reproduce

sushi-config.yaml:

canonical: http://example.org/repro
fhirVersion: 4.0.1
FSHOnly: true

input/fsh/repro.fsh:

Alias: $display = http://hl7.org/fhir/StructureDefinition/display

// 1. Repeating primitive, extension but NO value anywhere in the array.
Instance: NoValues
InstanceOf: Patient
Usage: #example
* name[0].given[0].extension[0].url = $display
* name[0].given[0].extension[0].valueString = "ext on given[0]"

// 2. Same, but one sibling entry has a value.
Instance: SomeValues
InstanceOf: Patient
Usage: #example
* name[0].given[0] = "Xavier"
* name[0].given[1].extension[0].url = $display
* name[0].given[1].extension[0].valueString = "ext on given[1]"

// 3. Non-repeating primitive, extension but no value.
Instance: NonRepeating
InstanceOf: Patient
Usage: #example
* name[0].family.extension[0].url = $display
* name[0].family.extension[0].valueString = "ext on family"

// 4. Repeating primitive with an id (not an extension) and no value.
Instance: NoValuesId
InstanceOf: Patient
Usage: #example
* name[0].given[0].id = "given-0"

Then sushi . and inspect name[0] in each generated instance.

Actual

// 1. NoValues — value array missing entirely
{"_given":[{"extension":[{"url":"…/display","valueString":"ext on given[0]"}]}]}

// 2. SomeValues — correct, arrays aligned and null-padded
{"given":["Xavier",null],"_given":[null,{"extension":[{"url":"…/display","valueString":"ext on given[1]"}]}]}

// 3. NonRepeating — correct, no array involved
{"_family":{"extension":[{"url":"…/display","valueString":"ext on family"}]}}

// 4. NoValuesId — same gap, with id rather than extension
{"_given":[{"id":"given-0"}]}

Expected

Cases 1 and 4 should null-pad the value array the same way case 2 does:

// 1.
{"given":[null],"_given":[{"extension":[{"url":"…/display","valueString":"ext on given[0]"}]}]}

// 4.
{"given":[null],"_given":[{"id":"given-0"}]}

Cases 2 and 3 are already correct and are included only as controls.

Spec reference

FHIR R4 §JSON Representation of Primitive Elements:

In the case where the primitive element may repeat, it is represented in two arrays. JSON null values are used to fill out both arrays so that the id and/or extension are aligned with the matching value in the first array

Note: when one of the repeating elements has no value, it is represented in the first array using a null. When an element has a value but no extension/id, the second array will have a null at the position of that element.

The second sentence covers this case directly: the repeating element has no value, so it should be represented in the first array using a null. The spec's worked example happens to show a mixed array, which may be why the all-absent case was missed.

Why it matters

We author SDC template-based $extract templates (sdc-questionnaire-templateExtractValue), where a directive is an extension on an element that deliberately has no value — the value is produced at extraction time. On repeating primitives such as HumanName.given and Address.line that is exactly case 1.

Uploading such a questionnaire to HAPI FHIR (tested on 8.10.0 and on hapi.fhir.org 8.11.16-SNAPSHOT) returns 201/200 with no OperationOutcome, but the stored resource has silently dropped those extensions.