STU3 Candidate

This page is part of the FHIR Specification (v1.8.0: STU 3 Draft). The current version which supercedes this version is 5.0.0. For a full list of available versions, see the Directory of published versions . Page versions: R4B R4 R3 R2

2.25.0 Element

FHIR Infrastructure

Work Group

Maturity Level: N/A

Ballot Status: STU 3

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 descendent 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 specialise the base type Resource.

Structure

Name	Flags	Card.	Type	Description & Constraints
Element	I		n/a	Base for all elements All FHIR elements must have a @value or children
id		0..1	id	xml:id (or equivalent in JSON)
extension		0..*	Extension	Additional Content defined by implementations
Documentation for this format

UML Diagram (Legend)

XML Template

<[name] xmlns="http://hl7.org/fhir" id="Internal id (e.g. like xml:id) (id)"> 
 <extension><!-- 0..* Extension Additional Content defined by implementations --></extension>
</[name]>

JSON Template

// complex types:
    
{
  "id" : "<id>", // Internal Id
  "extension" : [{//  Additional Content defined by implementations
    // from Element: extension (recursive)
    "url" : "<uri>", // R!  identifies the meaning of the extension
    "value[x]" : <*> // identifies the meaning of the extension
  }]  
}
// primitive types:
    
{
  "{name}" : "value",  // The primitive value
  "{name}_" : { // Special syntax for the id and extensions
    "id" : "<id>", // Internal Id
    "extension" : [{//  Additional Content defined by implementations
      // from Element: extension (recursive)
      "url" : "<uri>", // R!  identifies the meaning of the extension
      "value[x]" : <*> // identifies the meaning of the extension
    }]  
  }
}

Structure

Name	Flags	Card.	Type	Description & Constraints
Element	I		Element	Base for all elements All FHIR elements must have a @value or children
id		0..1	id	xml:id (or equivalent in JSON)
extension		0..*	Extension	Additional Content defined by implementations
Documentation for this format

UML Diagram (Legend)

XML Template

<[name] xmlns="http://hl7.org/fhir" id="Internal id (e.g. like xml:id) (id)"> 
 <extension><!-- 0..* Extension Additional Content defined by implementations --></extension>
</[name]>

JSON Template

// complex types:
    
{
  "id" : "<id>", // Internal Id
  "extension" : [{//  Additional Content defined by implementations
    // from Element: extension (recursive)
    "url" : "<uri>", // R!  identifies the meaning of the extension
    "value[x]" : <*> // identifies the meaning of the extension
  }]  
}
// primitive types:
    
{
  "{name}" : "value",  // The primitive value
  "{name}_" : { // Special syntax for the id and extensions
    "id" : "<id>", // Internal Id
    "extension" : [{//  Additional Content defined by implementations
      // from Element: extension (recursive)
      "url" : "<uri>", // R!  identifies the meaning of the extension
      "value[x]" : <*> // identifies the meaning of the extension
    }]  
  }
}

Constraints

ele-1: All FHIR elements must have a @value or children (expression : 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.

2.25.0.2 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.

2.25.0.3 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 'code' of type Coding:

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

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

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

2.25.0.4 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 object named "extension". Here is the representation for a property 'code' of type Coding:

  {
    "code" : {
      "_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 "_" appended. For example, a string property named "name" will be represented like this:

  {
    "name" : "[value of string]",
    "name_" : {
      "_id" : "[internal id]",
      "extension" : [
        ..extensions, if present...
      ]
    }
  }

The exact use of this pattern is described here.

2.25.0.5 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 use the internal id on the element in any way.

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

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