Define Extensions

Within Forge you can extend your profiles or elements with extensions created by yourself or others.

Extension registry

When using extensions, first consult the extension registry at http://hl7.org/fhir/extensibility-registry.html or http://simplifier.net/ to find already defined extensions that may be suitable for your needs. For example, extending a Patient profile with the place of birth can be done with an already existing extension found in in the HL7 extension registry.

New extension

If you cannot find an already defined extension you can make your own. From your project folder, go to New... and click New Extension (or Ctrl+E) to create a new extension. Forge shows warning messages in the lower section of the your screen highlighting the need to provide context information for this new extension.

Provide this information in Properties tab of the extension. Context Type indicates if the extension extends a Resource, Datatype, Mapping or another Extension. Give the exact context by clicking the button next to Expression.

This brings you to a new screen were you can select the specific resource or datatype where the extension is allowed to be placed. If you extend on the resource level you can click Select Resource. If you extend an element first select that element and then click Select Element. Depending on where the extension may be used, you can add more context information. You can provide a canonical URL, name and other relevant information in the properties section as well.

Extension profile

A new extension starts with one element containing a Value[X]. A Value[X] can contain all datatypes, most likely this needs to be constrained to a more specific datatype. This can be done in the Element Properties tab by selecting the wanted datatype(s). An extension containing more elements is called a complex extension. Elements can be added and removed from the extension profile with the use of the Add and Remove buttons. Added elements will be placed a level lower than the selected element.

Add the extension to the profile

Note that to add extensions, they need to be available in your project folder. So you will either need to create your own extension and save it to your project folder or add a package to your project folder.

Select the element in the Element Tree of your profile where you want to add the extension. Then, click Extend which is shown in the tool section above the element tree.

Forge shows the extensions that are available in your project folder.

Note that you can only select extensions that are compatible with your context (e.g. when extending an AdverseEvent resource you can only select an extension that has this resource as a context). By default, incompatible extensions are hidden, but you can also view incompatible extensions by clicking Filter to turn off filtering.

Add the extension to an element property

In addition to adding extensions to elements in the Element Tree of your profile you can also add extensions to some of the element properties.

If an element property can be extended then a details button is shown next to it. The color indicates if the element property has extensions:

• The element property does not have any extensions.

• The element property has one or more inherited extensions.

• The element property has one or more constrained extensions.

Extensions on element properties are supported for:

• Primitive types

  • string

  • markdown

• Data types

  • Address

  • Coding

  • CodeableConcept

  • ElementDefinition

  • UsageContext

• Element definition components

  • Base

  • Binding

    • Additional (R5)

  • Slicing

  • Type

You can view and edit extensions by clicking on the details button. To hide the extensions just click on the details buttton again. The following buttons are available to add a new extension to the element property:

The + button creates a new empty extension. The + Select... button opens a dialog showing the matching extensions that are available in your project folder. The button is only available for string and markdown properties and provides a shortcut for the translation extension.

For simple extensions only the value field is available.

Complex extensions contain elements that can either be value fields or child extension slices. If an element has a maximum cardinality of 1 then the element is always shown even if it is optional (you can leave it empty). If an element has a maximum cardinality higher than 1 then the element is a child extension slice and it will be listed in the Add slice dropdown box. You can add a new child extension slice by selecting the appropriate child extension slice name from the list.

Show Element Properties Details

To quickly show or hide extensions globally on all element properties you can use one of the following commands from the Options menu.

• On command only (Ctrl+1)

  Extensions are only displayed when you click the details button next to the element property.

• When Constrained (Ctrl+2)

  Extensions are displayed when they have one or more constraints. This is the default selection.

• When Constrained Or Inherited (Ctrl+3)

  Extensions are displayed when they have one or more constraints or when the base profile has defined one or more extensions.

• Element Id (Ctrl+4)

  Show or hide all Element Id fields for extensions. By default Element Id fields for extensions are hidden.