I'm trying to use the expression-based-criteria subscription extension to only fire a Subscription when a Consent is in

active

status where it was either previously

draft

OR has no previous state (directly created in

active

status). My filter expression valueString is

"%previous={} or %previous.status='draft'"

and the subscription criteria is

'Consent?status=active'

and it is working properly when a Consent goes from draft->active but it does not work when I directly create a Consent in

active

status. This leads me to believe that

%previous={}

is not doing what I expected. The docs note: https://www.medplum.com/docs/subscriptions/subscription-extensions#expression-based-criteria > Upon the creation of a resource, there won't be a previous version of the resource.

%previous

will be an empty object. How do we check if

%previous

is an empty object in the fhirpath expression? Maybe I want to use the [equivalent operator](https://build.fhir.org/ig/HL7/FHIRPath/#equivalent) ?

Not sure if this answers your question completely, but this is how you create a subscription on "create" https://www.medplum.com/docs/subscriptions/subscription-extensions#subscriptions-for-create-only-events

Going this route would require me to have 2 subscriptions for the scenario described so definitely not ideal but appreciate that there's a workaround available!

Yeah, I do think that a single sub is not supported for this case unfortunately

@node, have you tried

%previous.exists()

? This is a function that is in the FHIRPath spec. You can see an example here wrt. write-constraints https://www.medplum.com/docs/access/access-policies#write-constraints

Yup tried that and it does not seem to be working either I dug a bit into the fhirpath implementation and poked around - I think there might be a bug in the

exists

&

empty

function implementations Opened this issue to track https://github.com/medplum/medplum/issues/3807

Thanks for the issue and PR @node ! We'll take a look