Reverse chaining not returning any results


I am new to the FHIR logic and I am trying to create some API calls that will give me the Patients that match certain criteria. Specifically, I am trying to create some API calls that use reverse chaining, but they don’t return any results and I am not sure why.

For example, I want to get all Patients that have a specific cancer type, with code “781382000”. Thus my request is as follows: [base]/fhir-server/api/v4/Patient?_has:Observation:patient:code=781382000.
However I get 0 results. Is there something wrong with my request?

The following request gives me the correct observations: [base]/fhir-server/api/v4/Observation?code=781382000, but I need the patients…

In case it helps, here is my Observation resource:

            <Observation xmlns="">
                <id value="ob-1"/>
                <status value="final"/>
                        <system value=""/>
                        <code value="781382000"/>
                        <display value="X cancer"/>
                    <reference value="Patient/ABCD123"/>
                    <type value="Patient"/>
                    <reference value="Organization/org-1"/>
                    <type value="Organization"/>

And my Patient resource:

            <Patient xmlns="">
                <id value="ABCD123"/>
                <extension url="">
                    <valueCode value="0"/>
                        <text value="Identifier"/>
                    <value value="ABCD123"/>
                <gender value="..."/>
                <birthDate value="..."/>
                    <reference value="Organization/organization-id-01"/>


Does the server support reverse chaining? What gets echoed back in the “self” link?

Yes it does.

My mistake was on the way I was handling the patient ids, thus all references to them were incomplete.

In order for reverse chaining to work as expected, I must upload all Patient resources using a “PUT” request at [base]/Patient<p_id>,
while I was uploading them using a “POST” request at [base]/Patient