Attempting to access dose of a Medication Request returns null

[brynrhodes]

We have CQL that is attempting to access the MedicationRequest.dosageInstruction.doseAndRate.dose element. The dose element in this case is a Choice<SimpleQuantity, Range>, but the actual element in the JSON is spelled doseQuantity, so when the choice selection code looks for the JSON property:

cql-exec-fhir/src/fhir.js

Line 173 in f175661

const choiceProperty = choicePropertyName(element, choice);

It's actually looking for doseSimpleQuantity, which doesn't exist, so returns null.

We are trying to workaround this particular case by sneakily modifying the ModelInfo, but this is a general problem with the subtyped elements (when they are part of a choice) in FHIR.

[brynrhodes]

We are trying the following workaround, adding to this function:

https://github.com/cqframework/cql-exec-fhir/blob/master/src/fhir.js#L155

const choicePropertyName = function(element, choice) {
  if (choice === 'SimpleQuantity') choice = 'Quantity'
  return `${element.name}${choice.name[0].toUpperCase()}${choice.name.slice(1)}`;
};

Something like that, though I have no idea if that's valid JS, web devs are working on it now

[cmoesel]

That approach seems reasonable -- although not general enough to cover all potential use cases. Ideally we would have a more generic solution. Part of the difficulty, however, is that sometimes it's appropriate to use the base type (as in SimpleQuantity, a profile on Quantity) but sometimes it is not (as in Age, a specialization of Quantity). A robust solution will need to take this into account.

[brynrhodes]

FYI, this did work, and totally agree, it's not general enough for a complete solution, but it does address the specific case of SimpleQuantity (so long as the underlying field is in fact named XXXQuantity, if the underlying field was named XXXSimpleQuantity, it would actually break that, but I'm not aware of that pattern in the 4.0.1 schema).