Tie a StructureDefinition to an Extension

[Brad Solomon]

I'm looking for a simple hello-world example of creating a custom Extension and linking its .url field to a StructureDefinition.

Background: we are using the HAPI FHIR v6.6.2 Plain Server and we expose the ResearchStudy FHIR resource, with base URL (for this example) hosted at https://foo.com/fhir.

We would like to add a custom Extension to the ResearchStudy resource:

- Extension name = NumUniqueDatasets
- Type = int
- Cardinality: 0..1

Example response for /fhir/ResearchStudy/1:

    {
      "resourceType": "ResearchStudy",
      "id": "1",
      "extension": [ {
        "url": "https://foo.com/fhir/StructureDefinition/ResearchStudy-NumUniqueDatasets",
        "valueInteger": 20
      } ],
      "status": "active"
      ...
    }

I know that I need some Java code to create a new extension and add it to the study:

    Extension ext = new Extension();
    ext.setValue(new IntegerType(20));
    myResearchStudy.addExtension(ext);

But the HAPI FHIR docs on Profiles and Extensions (https://hapifhir.io/hapi-fhir/docs/model/profiles_and_extensions.html) don't give a more realistic example that actually shows how to serve up the StructureDefinition itself at https://foo.com/fhir/StructureDefinition/ResearchStudy-NumUniqueDatasets.

How do I do this using HAPI FHIR? How do I create the StructureDefinition and tie my Extension instance back to it? Is there a way to auto-set the Extension URL without needing to explicitly call ext.setUrl()?

[Brad Solomon]

To elaborate on my question here in a few ways, in case it is unclear:

• To expose our custom StructureDefinition at "https://foo.com/fhir/StructureDefinition/ResearchStudy-NumUniqueDatasets" (under our base server URL), do I need to register a new resource provider for org.hl7.fhir.r5.model.StructureDefinition as I would for a ResearchStudy resource? Or is there some other way of doing this?
• I see some references in the Extension class source code about auto-creating this.url (https://hapifhir.io/hapi-fhir/apidocs/hapi-fhir-structures-r5/src-html/org/hl7/fhir/r5/model/Extension.html#line-106), but this doesn't seem to be documented anywhere nor is it clear what the behavior here is actually supposed to be. Is there a way to auto-create the Extension.url without explicitly needing to call .setUrl()?