Add Reference fields that can be equijoined

[rbrush]

FHIR References store a relative URL to the item they are referencing. This is often of the form Patient/<patient_id_here>. The downside to this is that the reference field cannot be used in simple equality-based joins, since the relative prefix (like "Patient/" or "Encounter/" is present in the reference string.

Therefore, the we should consider additional fields in the Reference structure -- perhaps "reference_id" and "reference_type" -- where the former can be used for equality-based joins and the later defines the type of resource being referenced.

[niquola]

We use following reference structure:

{id: "pt-1", resourceType: "Patient"}

There was idea unified with union type:

{ Patient: 'pt-id' [, resourceType: "Patient" ] / * optional */ }

See discussion here fhir-fuel/fhir-fuel.github.io#4

[niquola]

What nice about {id: .., resourceType:... } - that you can replace ref with resource and have same id and rt attrs (actual for graphql like results)

[rbrush]

Something like {id: .., resourceType:.. } makes sense to me. I think we might need to make it {resourceId: .., resourceType:.. }, since the "id" field is reserved for the ancestor Element. [1]

If {resourceId: .., resourceType:.. } seems okay, I'll create a PR with that change.

[1]
https://www.hl7.org/fhir/element.html#Element

[rbrush]

This is incorporated into PR #9