I want to filter Encounters based on their first or last Encounter.location[].
FHIR REST api spec has only 1 search param related to Encounter location value: location and it doesn't specify the search behaviour in case when Cardinality is greater than 1.
I was hoping for a modifier suffix like :first or :last to support location:first, location:last i.e
{base_url}/Encounter?location:last=Location/123
but there is no such option and I don't see a way to add custom parameter modifiers.
Is there a preferred way than adding custom search params like location-last, location-first ?
Thanks!
The first thing to be aware of is that order is only significant when the base FHIR specification declares it to be significant - which it doesn't for Encounter.location. As a result, you should expect the locations to be listed in arbitrary order and the 'first' location won't mean anything special. The period of time when a location is/was relevant is conveyed by Encounter.location.period, not ordering within the collection. It's non-conformant to enforce any rules about ordering when FHIR hasn't defined any.
There are currently no qualifiers to support searching on a specific repetition. However, it is possible to define custom search parameters that can be tied to a specific repetition. E.g. If you wanted to only search on a patient's "first" name as opposed to all given names. Be aware that defining custom search parameters is only useful if you're in an environment where you can nudge both clients and servers to support the custom parameter.
You could submit a change request (use the link at the bottom of any page in the spec) proposing the :first and :last modifiers for inclusion in R5 of FHIR, but you'd have to provide several examples of places where it would be useful (constrained to places where order is actually significant)