GraphQL resource instance level query

[codyebberson]

https://hl7.org/fhir/graphql.html#invoking

[base]/[Type]/[id]/$graphql

A Resource Instance level the query assumes a single resource is in scope, and just queries on the data available in the given resource.

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Implementation notes:

This will mostly be in packages\fhir-router\src\graphql\graphql.ts

We currently only maintain one root schema:

let rootSchema: GraphQLSchema | undefined;

Which is lazily built on the first graphql operation.

We will need to create additional schemas for each FHIR resource type. That should not be as bad as it sounds. We already build a GraphQLOutputType for each resource type. So creating the schema should be something like:

let rootSchema: GraphQLSchema | undefined;

const instanceSchemas = new Map<string, GraphQLSchema>();

function getInstanceSchema(resourceType: string): GraphQLSchema {
  let result = instanceSchemas.get(resourceType);
  if (!result) {
    result = buildInstanceLevelSchema(resourceType);
    instanceSchemas.set(resourceType, result);
  }
  return result;
}

function buildInstanceLevelSchema(resourceType: string): GraphQLSchema {
  return new GraphQLSchema({
    query: buildGraphQLOutputType(resourceType),
  });
}

(untested)

And then add a route in packages\fhir-router\src\fhirrouter.ts

Similar to the existing GraphQL route:

    this.router.add('POST', '$graphql', graphqlHandler);

Except this will be an instance level route:

    this.router.add('POST', ':resourceType/:id/$graphql', graphqlHandler);

I don't know if it would be easier to reuse the existing graphqlHandler or to create a new similar one for instance level. That will require some quick experimentation.