convert_release — Convert a resource between FHIR releases, and read the loss report

Run with:

cargo run --example convert_release --features "r4 r5"

fhir::convert::between converts a document's JSON wire form from one release to another, driven by both releases' element metadata, and returns a LossReport naming everything it changed or discarded. The report is the point: serde alone would silently drop an element the target release does not have, which is the failure mode a converter exists to prevent.

This example converts an R5 Observation down to R4 twice — once with a shape both releases agree on (the report comes back empty, and that emptiness is an assertion), and once carrying triggeredBy, an element R5 added, so the downgrade has something to report.

The program#

use fhir::convert;
use fhir::r4::R4;
use fhir::r5::R5;

fn main() {
    // A wire document, as it might arrive from an R5 server. `between` works
    // on the JSON form, so nothing needs to be deserialized first — and the
    // `resourceType` tag in the document is what it converts by.
    let stable = serde_json::json!({
        "resourceType": "Observation",
        "id": "temp-1",
        "status": "final",
        "code": { "text": "Body temperature" },
        "valueQuantity": { "value": 36.6, "unit": "C" }
    });

    // R5 -> R4, on a shape the two releases agree about.
    let down = convert::between::<R5, R4>(&stable);
    assert!(down.report.is_lossless(), "unexpected loss:\n{}", down.report);
    println!("stable Observation: converted losslessly");

    // The result is ready for the target release's model.
    let as_r4: fhir::r4::resources::Resource =
        serde_json::from_value(down.value).expect("valid R4 after conversion");
    if let fhir::r4::resources::Resource::Observation(obs) = as_r4 {
        println!("R4 status: {}", obs.status.code());
    }

    // The same Observation carrying `triggeredBy` — an element R5 added, which
    // R4 has no place for. The conversion still succeeds; the report says what
    // it had to drop, instead of dropping it silently.
    let mut r5_only = stable.clone();
    r5_only["triggeredBy"] = serde_json::json!([
        { "observation": { "reference": "Observation/base-1" }, "type": "reflex" }
    ]);

    let lossy = convert::between::<R5, R4>(&r5_only);
    assert!(!lossy.report.is_lossless());
    println!("\nR5-only element on the way to R4 — the report:");
    println!("{}", lossy.report);

    // `discarded_data` separates real loss from warnings about data kept.
    println!(
        "discarded data: {} ({} loss(es) recorded)",
        lossy.report.discarded_data(),
        lossy.report.len(),
    );
}

Source: fhir/examples/convert_release.rs in the repository.