Stream: fhirpath
Topic: Selecting from distinct collection in a specified order
Morten Ernebjerg (Mar 12 2021 at 16:27):
Hi :wave: I was playing around with FHIRPath and trying to come up with an expression for reducing a CodeableConcept to a string. The logic I want involves preferences, e.g.
- if there are codes that are use selected, take the first display value from those
- otherwise, if there are codes from code systems I know, take the display string of the first of those
- otherwise take
CodeableConcept.text
The generic logic here is I have several expressions that each yield a collection (possibly empty) and I want to go through those collections in a specified order and pick some element from the first non-empty collection. If these were arrays, I would concatenate them in the required order and take the first element, but the union operator does not guarantee to maintain the ordering. I can think of a few very ugly solutions - are there any elegant ones?
ryan moehrke (Mar 12 2021 at 16:28):
my first thought is iif() chaining but that could be quite messy and long for all the processing you want to do
Morten Ernebjerg (Mar 12 2021 at 16:36):
Yup, that should work. I did consider it, but I was hoping for something that would avoid this sort of nesting and be more functional-style (a hope I could not make come true myself by reading the spec :smirk: )
Bryn Rhodes (Mar 12 2021 at 18:13):
Using a unions of firsts should work, but it would be long. It's a prime candidate for a function, but given that you're looking for "code systems you know" it couldn't be a standard function (unless everyone agreed what that meant).
ryan moehrke (Mar 12 2021 at 18:16):
but how do you differentiate between the three preferred options in that union'd set?
Bryn Rhodes (Mar 12 2021 at 18:26):
Mmmm, yes, that's true, sorry. Yep, iif. We considered coalesce(), like CQL has, but opted against it since it was shorthand for iif.
Brian Postlethwaite (Jun 06 2021 at 10:05):
(coding.where(system="blah").code | coding.where(system="blah_2nd").code | coding.where(system="3rd_system").code ).first()
Last updated: Apr 12 2022 at 19:14 UTC
