STU 3 Ballot
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| FHIR Infrastructure Work Group | Maturity Level: 3 | Ballot Status: STU 3 |
Many of the defined elements in a resource are references to other resources. Using these references, the resources combine to build a web of information about healthcare.
Resources contain two types of references:
References are always defined and represented in one particular direction - from one resource (source) to another (target). References are provided as a URL, which may either be absolute or relative. Resolving the references is discussed below.
The corresponding reverse relationship from the target to the source exists in a logical sense, but is not represented explicitly in the resource. For external references, navigating these reverse relationships requires some external infrastructure to track the relationship between resources (the REST API provides one such infrastructure by providing the ability to search the reverse relationship by naming search parameters for the references, and by providing support for reverse includes).
Because resources are processed independently, relationships are not considered to be transitive. For example, if a Condition resource references a particular Patient as its subject, and references a Procedure resource as its cause, there is no automatic rule or implication that the procedure has the same patient for its subject. Instead, the subject of the procedure must be established directly in the Procedure resource itself. Another way to state this is that the context of the subject is not "inherited", nor does it "conduct" along the relationship to procedure. The only exception to this is the case of contained resources (see below). Note that in practice, the relationships need to describe a logical and coherent record, and in the case of the Condition and Procedure described here, they would usually be required to have the same patient for their subjects.
In a resource, references are represented with a reference and a text description. The reference is the key element - resources are identified and addressed by their URL. The actual reference looks like this:
Structure
| Name | Flags | Card. | Type | Description & Constraints |
|---|---|---|---|---|
  Reference | ΣI | Element | A reference from one resource to another SHALL have a local reference if the resource is provided inline | |
  reference | ΣI | 0..1 | string | Relative, internal or absolute URL reference |
 display | Σ | 0..1 | string | Text alternative for the resource |
| Documentation for this format | ||||
UML Diagram (Legend)
XML Template
<[name] xmlns="http://hl7.org/fhir"><!-- from Element: extension --> <reference value="[string]"/><!--
0..1 Relative, internal or absolute URL reference --> <display value="[string]"/><!-- 0..1 Text alternative for the resource --> </[name]>
JSON Template
{
// from Element: extension
"reference" : "<string>", // C? Relative, internal or absolute URL reference
"display" : "<string>" // Text alternative for the resource
}
Turtle Template
@prefix fhir: <http://hl7.org/fhir/> . [ # from Element: Element.extension fhir:Reference.reference [ string ]; # 0..1 Relative, internal or absolute URL reference fhir:Reference.display [ string ]; # 0..1 Text alternative for the resource ]
Changes since DSTU2
| Reference | No Changes |
See the Full Difference for further information
Structure
| Name | Flags | Card. | Type | Description & Constraints |
|---|---|---|---|---|
| Reference | ΣI | Element | A reference from one resource to another SHALL have a local reference if the resource is provided inline | |
| reference | ΣI | 0..1 | string | Relative, internal or absolute URL reference |
| display | Σ | 0..1 | string | Text alternative for the resource |
| Documentation for this format | ||||
XML Template
<[name] xmlns="http://hl7.org/fhir">
JSON Template
{
// from Element: extension
"reference" : "<string>", // C? Relative, internal or absolute URL reference
"display" : "<string>" // Text alternative for the resource
}
Turtle Template
@prefix fhir: <http://hl7.org/fhir/> . [ # from Element: Element.extension fhir:Reference.reference [ string ]; # 0..1 Relative, internal or absolute URL reference fhir:Reference.display [ string ]; # 0..1 Text alternative for the resource ]
Constraints
: reference.startsWith('#').not() or (reference.substring(1).trace('url') in %resource.contained.id.trace('ids')))Notes:
reference element contains a url that is either
Bundle.entry.fullUrl (see Resolving References in Bundles)
((http|https)://([A-Za-z0-9\\\/\.\:\%\$])*)?(Account|ActivityDefinition|AllergyIntolerance|Appointment|AppointmentResponse|AuditEvent|Basic|Binary|BodySite|Bundle|CarePlan|CareTeam|Claim|ClaimResponse|ClinicalImpression|CodeSystem|Communication|CommunicationRequest|CompartmentDefinition|Composition|ConceptMap|Condition|Conformance|Consent|Contract|Coverage|DataElement|DecisionSupportServiceModule|DetectedIssue|Device|DeviceComponent|DeviceMetric|DeviceUseRequest|DeviceUseStatement|DiagnosticReport|DiagnosticRequest|DocumentManifest|DocumentReference|EligibilityRequest|EligibilityResponse|Encounter|Endpoint|EnrollmentRequest|EnrollmentResponse|EpisodeOfCare|ExpansionProfile|ExplanationOfBenefit|FamilyMemberHistory|Flag|Goal|Group|GuidanceResponse|HealthcareService|ImagingManifest|ImagingStudy|Immunization|ImmunizationRecommendation|ImplementationGuide|Library|Linkage|List|Location|Measure|MeasureReport|Media|Medication|MedicationAdministration|MedicationDispense|MedicationOrder|MedicationStatement|MessageHeader|NamingSystem|NutritionRequest|Observation|OperationDefinition|OperationOutcome|Organization|Patient|PaymentNotice|PaymentReconciliation|Person|PlanDefinition|Practitioner|PractitionerRole|Procedure|ProcedureRequest|ProcessRequest|ProcessResponse|Provenance|Questionnaire|QuestionnaireResponse|ReferralRequest|RelatedPerson|RiskAssessment|Schedule|SearchParameter|Sequence|Slot|Specimen|StructureDefinition|StructureMap|Subscription|Substance|SupplyDelivery|SupplyRequest|Task|TestScript|ValueSet|VisionPrescription)\/[A-Za-z0-9\-\.]{1,64}(\/_history\/[A-Za-z0-9\-\.]{1,64})?
display, if populated, does not have identical content to the Resource.text of the referenced resource. The purpose is to
identify what's being referenced, not to more fully describe itA relative reference to the Patient "034AB16" in an element named context on a FHIR RESTful server:
<context>
<reference value="Patient/034AB16" />
</context>
An absolute reference to a Structure Definition in an element named profile:
<profile>
<reference value="http://fhir.hl7.org/svc/StructureDefinition/c8973a22-2b5b-4e76-9c66-00639c99e61b" />
</profile>
Note that HL7 has not yet actually created a profile registry, nor decided on a URL for it.
A short display text that provides a human-readable identification of the resource may be provided:
<custodian>
<reference value="Organization/123" />
<display value="HL7, Inc" />
</custodian>
This text can be used by a system that is unable to resolve the reference to an actual resource.
Note that some elements have the type uri instead of Reference.
URIs may point to resources, to elements inside a resource by their id property, or (most often)
to some other content that is not a resource. The Reference type is only used to refer to resources
directly, by their logical id.
Many resource types have a defined element "url" which is the canonical URL that always identifies the resource. These include all the conformance and knowledge resources (most of the resources not found in the Patient Compartment).
The canonical URL remains the same when the resource is copied from server to server, while the logical id of the resource - it's local identifier - usually changes as the resource is copied. The canonical URL serves as a stable logical identifier for the conformance artifact, and is the preferred way to reference a conformance or knowledge resource. The canonical URL is also the location where the master copy of the artifact is found.
Systems resolving references to resources that might have cannonical URLs SHOULD first try to resolve the reference using the cannonical URL, and then fall back to direct resolution against the URL if a local version of the canonical resource cannot be found. This approach is safe because the local version cannot be a different artifact that the master copy, though implementations will need to make appropriate arrangements regarding the currency of their local copy of the artifact.
In some circumstances, the content referred to in the resource reference does not have an independent existence apart from the resource that contains it - it cannot be identified independently, and nor can it have its own independent transaction scope. Typically, such circumstances arise where resources are being assembled by a secondary user of the source data, such as a middleware engine. If the data available when the resource is constructed does not include record keys or absolute identification information, then a properly identified resource cannot be assembled, and even if an arbitrary identification was associated with it, the resource could never be the subject of a transaction outside the context of the resource that refers to it.
For example, consider a situation where an interface engine is creating a Condition
record on a patient from an HL7 v2 message, and the only information about the primary surgeon is her
first name and last name (REL-7.2 & RES-7.3). In the absence of a controlled practitioner directory,
this is not enough information to create an identified Practitioner
resource since more than one practitioner might have the same name.
In these circumstances, the resource is placed directly in-line in the resource. This SHOULD NOT be done when the content can be identified properly, as once the identification is lost, it is extremely difficult (and context dependent) to restore it again.
An example of a contained resource:
<Composition xmlns="http://hl7.org/fhir">
<extension>...</extension>
<text>...</text>
<contained>
<Organization>
<id value="org1"/>
<!-- whatever information is available -->
</Organization>
</contained>
<information>
<!-- other attributes -->
<custodian>
<reference value="#org1" />
</custodian>
<!-- other attributes -->
<information>
</Composition>
The same example in JSON:
{ "resourceType" : "Composition",
"extension" : { ... },
"text" : { .. },
"contained: [
{
"resourceType" : "Organization",
"id" : "org1",
.. whatever information is available ...
} ]
"information: {
... other attributes ...
"custodian" : {
"reference" : "#org1"
}
... other attributes ...
}
}
Design Note: Contained resources are still a reference rather than being inlined directly into the element that is the reference (e.g. "custodian" above) to ensure that a single approach to resolving resource references can be used. Though direct containment would seem simpler, it would still be necessary to support internal references where the same contained resource is referenced more than once. In the end, all that it would achieve is creating additional options in the syntax. For users using XPath to process the resource, the following XPath fragment resolves the internal reference:
ancestor::f:*[not(parent::f:*)]/f:contained/*[@id=substring-after(current()/f:reference/@value, '#')]
Some notes about use and interpretation of contained resources:
contained element SHALL NOT have extensions on it (though contained resources can still contain extensions)DSTU Note: It might be necessary to allow for the reverse (that is, for a contained resource to reference the container). However this would cause many difficult consequences on the API. Feedback on this issue is welcome here
Resources that are contained inline do not "inherit" context from their parent resource. For instance, if the parent resource contains a "subject", and the contained resource also has a "subject" element defined, there is no implication that the contained resource has the same subject as the parent resource.
Resources can only be contained in other resources if there is a reference from the resource to the contained resource. This is intended to ensure that the meaning of the contained resource is clear, and that there is no confusion as to its significance.
DSTU Note: There are some identified use cases where it would be useful to include resources that refer to the contained resource rather than the container referring to the contained resource, but this has a series of structural ramifications for the API. Whether these can be resolved is an open issue for investigation during the period of trial use.
Feedback is welcome here
© HL7.org 2011+. FHIR STU3 Ballot (v1.6.0-9663) generated on Thu, Aug 11, 2016 17:07+1000. QA Page
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