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• Element
• Detailed Descriptions
• Extensions

2.28.0 Element

FHIR Infrastructure Work Group

Maturity Level: Normative

Standards Status: Normative

This page has been approved as part of an ANSI standard. See the Infrastructure Package for further details.

The base definition for all elements contained inside a resource. All elements, whether defined as a Data Type (including primitives) or as part of a resource structure, have this base content:

• Extensions
• An internal id

There are 3 kinds of descendant types that specialize Element:

• Primitive data types, that add a primitive value property of the specified type
• Complex data types, that add their own children (all of which are also elements)
• BackboneElement, A specialization that adds modifierExtension, which is the super-type of all the element types defined in resource definitions (e.g. Patient.contact)

Note that resources themselves all specialize the base type Resource.

2.28.0.1 Content

 

Constraints

id	Level	Location	Description	Expression
ele-1	Rule	(base)	All FHIR elements must have a @value or children	hasValue() or (children().count() > id.count())

This constraint exists to reduce syntactical variation in resource contents. If an element has no children, then it is always omitted from the resource, as opposed to optionally present without any content.

2.28.0.2 Representation of Element

As the base type for all elements included in a resource, Element is an important structural element of FHIR. Even the primitive types inherit the base features and representation rules that apply to the Element type.

2.28.0.3 XML Representation

Elements are represented by an XML element. The name of the element comes from the context in which it is used, not from the type. The internal id is represented as an attribute (similar to xml:id, but see below about scope). Extensions are represented as XML elements. Here is the representation for an element 'use' of type Coding:

  <use id="[internal id]">
    <extension url="..."/>
      ... if there are any extensions
    <extension>
    .. elements of Coding type...
  </use>

Primitive types have the same representation; the actual primitive value appears as an XML attribute named value on the XML element. For example, a code property named "status" will be represented like this:

  <status id="[internal id]" value="[value of code]"">
    <extension url="..."/>
      ... if there are any extensions
    <extension>
  </status>

2.28.0.4 JSON Representation

Elements (except for primitive types, see below) are represented by a JSON object property. The name of the property comes from the context in which it is used, not from the type. The internal id is represented as a JSON string property named "id". Extensions are represented in a JSON array of objects named "extension". Here is the representation for a property 'use' of type Coding:

  {
    "use" : {
      "id" : "[internal id]",
      "extension" : [
        ..extensions, if present...
      ],
      .. properties of Coding type...
    }
  }

Primitive types are represented differently; the actual primitive value appears as a JSON string or number property. If an internal id or extensions are present, they appear in a JSON object with the name of the primitive value property with "_" prepended. For example, a code property named "status" will be represented like this:

  {
    "status" : "[value of code]",
    "_status" : {
      "id" : "[internal id]",
      "extension" : [
        ..extensions, if present...
      ]
    }
  }

The exact use of this pattern is described here.

2.28.0.5 Internal Id Scope

The id property of the element is defined to allow implementers to build implementation functionality that makes use of internal references inside the resource. This specification does not define any general use for the internal id, though some resources (e.g. StructureDefinition) and extensions (e.g. originalText, narrativeLink) make use of it.

The internal id is unique within the scope of the resource that contains it. Specifically, this means:

• The id SHALL be unique within a given resource
• The uniqueness boundary extends into contained resources. i.e. a contained resource cannot have the same id as any element in the resource that contains it or any other contained resource
• The uniqueness boundary is broken at Bundle.entry.resource and Parameters.parameter.resource, since these are elements that aggregate different resources
• The id element does not have extensions itself

These rules ensure that there is no need to change internal identifiers while exchanging resources.