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2.30.0 Element

FHIR Infrastructure Work GroupMaturity Level: 5Ballot Status: Trial Use

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:

There are 3 kinds of descendent types that specialize Element:

Note that resources themselves all specialize the base type Resource.

Structure

NameFlagsCard.TypeDescription & Constraintsdoco
.. Element In/aBase for all elements
All FHIR elements must have a @value or children
... id 0..1stringxml:id (or equivalent in JSON)
... extension 0..*ExtensionAdditional Content defined by implementations

doco Documentation for this format

XML Template

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

JSON Template

// complex types:

{doco
  "id" : "<string>", // 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:

{doco
  "{name}" : "value",  // The primitive value
  "{name}_" : { // Special syntax for the id and extensions
    "id" : "<string>", // 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

NameFlagsCard.TypeDescription & Constraintsdoco
.. Element IElementBase for all elements
All FHIR elements must have a @value or children
... id 0..1stringxml:id (or equivalent in JSON)
... extension 0..*ExtensionAdditional Content defined by implementations

doco Documentation for this format

XML Template

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

JSON Template

// complex types:

{doco
  "id" : "<string>", // 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:

{doco
  "{name}" : "value",  // The primitive value
  "{name}_" : { // Special syntax for the id and extensions
    "id" : "<string>", // 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 : hasValue() | (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.

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.

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 are any extensions
    <extension>
    .. elements of Coding type...
  </code>

Primitive types have the same representation; the actual primitive value appears as an XML attribute 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 are any extensions
    <extension>
  </code>

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 '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 "_" prepended. 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.

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.