Extensions

FHIR-Schema defines optional extensions. These extensions allow to model data not representable in FHIR otherwise. However when using these extensions it is not possible to convert FHIR-Schema into equivalent StructureDefinition.

Enable Fhir-schema extensions

To enable FHIR-Schema extensions, the following key must be present at the top level of a FHIR-Schema definition:

ALLOW_FHIR_SCHEMA_FHIR_INCOMPATIBLE_EXTENSIONS: true

Any extensions are allowed ONLY for schemas with

derivation: specialization

If any extensions found in the schema while the key described above is not present, the schema is rejected as invalid.

Available extensions

any

Any element (including the top-level one) can contain the

any: true

property. If this property is present, the contents of the corresponding subtree of a resource is not validated.

Usage example

ALLOW_FHIR_SCHEMA_FHIR_INCOMPATIBLE_EXTENSIONS: true
url: schema-1
elements:
  knownElement:
    any: true

Limitations

The any property exlusive with any other properties:

• If there are multiple schemas for an element, where at least one specifies the any property, it will be rejected as invalid.
• If there are multiple schemas for an element, where at least one specifies any other property, it will be rejected as invalid.

For example, this is prohibited:

ALLOW_FHIR_SCHEMA_FHIR_INCOMPATIBLE_EXTENSIONS: true
url: schema-1
elements:
  knownElement:
    any: true
---
ALLOW_FHIR_SCHEMA_FHIR_INCOMPATIBLE_EXTENSIONS: true
url: schema-2
base: schema-1
elements:
  knownElement:
    type: string

additionalProperties

Any element (including the top-level one) can contain the

additionalProperties:
  # <schema for additional properties>

property. Any key which does not correspond to any key defined in the elements property is validated using the schema supplied under the additionalProperties property.

Usage example

The following example means that any property except knownElement and _knownElement is allowed and valid if it is a string value.

Schema:

ALLOW_FHIR_SCHEMA_FHIR_INCOMPATIBLE_EXTENSIONS: true
url: schema-1
elements:
  knownElement:
    type: integer
additionalProperties:
  type: string

Valid resource:

knownElement: 1
_knownElement:
  id: test
unknownElement: stringValue
_unknownElement: test2
______: test3

Invalid resources:

knownElement: 1
unknownElement: 2
---
knownElement: 1
_knownElement: test

Limitations

FHIR reserves the underscore (_) characters in property names. FHIR JSON representation separates any element in two parts:

• propertyName
• _propertyName

Property resolution behavior:

• If the property does not start with underscore and is defined in elements, it is interpreted as a normal part of the corresponding element.
• If the property starts with a single underscore, and the corresponding property name without underscore is defined in elements, it is interpreted as an underscored part of the corresponding element.
• Otherwise, it is interpreted as a normal part of an element specified under the additionalProperties property (regardless of underscores).

Extensions reserved for future use

The following properties are reserved for future use:

• properties
• additionalElements