schemeref

A <schemeref> element provides a reference to another scheme. Typically, the referenced scheme defines a base set of controlled values extended by the current scheme. The values in the referenced scheme are merged with the current scheme; the result is equivalent to specifying all of the values in a single map.

Contains

Note: These models represent only the default document types distributed by OASIS. Actual content models will differ with each new document type.

Doctype	Content model
subjectScheme	( (topicmeta) (optional) then (data or data-about) (any number) )

Contained by

Doctype	Content model
subjectScheme	subjectScheme

Inheritance

- map/topicref subjectScheme/schemeref

Example: Extending a category with more specific values

Figure 1. The map baseOS.ditamap

<subjectScheme>
  <subjectdef keys="os" navtitle="Operating system">
    <subjectdef keys="linux" navtitle="Linux">
      <subjectdef keys="redhat" navtitle="RedHat Linux"/>
      <subjectdef keys="suse"   navtitle="SuSE Linux"/>
    </subjectdef>
    <subjectdef keys="mswin" navtitle="Windows"/>
    <subjectdef keys="zos"   navtitle="z/OS"/>
  </subjectdef>
  <enumerationdef>
    <attributedef name="platform"/>
    <subjectdef keyref="os"/>
  </enumerationdef>
</subjectScheme>

Figure 2. Second map that references baseOS.ditamap

Because a scheme establishes relationships between subjects rather than a contextual navigation structure, new relationships can be added to existing subjects. In particular, the referencing scheme can extend an enumeration by adding new relationships to existing subjects that belong to the enumeration. For instance, a scheme could extend the baseOS.ditamap scheme shown above by adding Macintosh OS as a child of the existing os subject and adding special versions of Windows under the existing mswin subject:

<subjectScheme>
  <schemeref href="baseOS.ditamap"/>
  <subjectdef keyref="os">
    <subjectdef keys="macos" navtitle="Macintosh"/>
    <subjectdef keyref="mswin">
      <subjectdef keys="winxp"  navtitle="Windows XP"/>
      <subjectdef keys="winvis" navtitle="Windows Vista"/>
    </subjectdef>
  </subjectdef>
</subjectScheme>

The references to the subjects defined by the base scheme use the keyref attribute to avoid duplicate definitions of the keys.

Figure 3. Resulting scheme

The result of merging the extension scheme with the base scheme is exactly the same as the following single scheme.

<subjectScheme>
  <subjectdef keys="os" navtitle="Operating system">
    <subjectdef keys="linux" navtitle="Linux">
      <subjectdef keys="redhat" navtitle="RedHat Linux"/>
      <subjectdef keys="suse"   navtitle="SuSE Linux"/>
    </subjectdef>
    <subjectdef keys="macos" navtitle="Macintosh"/>
    <subjectdef keys="mswin" navtitle="Windows">
      <subjectdef keys="winxp" navtitle="Windows XP"/>
      <subjectdef keys="win98" navtitle="Windows Vista"/>
    </subjectdef>
    <subjectdef keys="zos"   navtitle="z/OS"/>
  </subjectdef>
  <enumerationdef>
    <attributedef name="platform"/>
    <subjectdef keyref="os"/>
  </enumerationdef>
</subjectScheme>

Because the extended baseOS scheme bound the os subject to the platform attribute, the extension scheme doesn't provide that binding. The controlled values added by the extension to the hierarchy for the os subject become part of the enumeration bound to the platform attribute.

Example: Extending a category upwards

A category can also be extended upward. For instance, an extension scheme could create a Software category that includes operating systems as well as applications.

<subjectScheme>
  <schemeref href="baseOS.ditamap"/>
  <subjectdef keys="sw" navtitle="Software">
    <subjectdef keyref="os"/>
    <subjectdef keys="app" navtitle="Applications">
      <subjectdef keys="apacheserv" navtitle="Apache Web Server"/>
      <subjectdef keys="mysql"      navtitle="MySQL Database"/>
    </subjectdef>
  </subjectdef>
</subjectScheme>

If the extended baseOS scheme defines the binding of the os subject with the platform attribute, the app subjects provided by the extension scheme aren't subordinate to the os subject and thus don't become part of that enumeration. To leave open the possibility of upward extension of an enumeration, the content provider should define the controlled values in one scheme and define the binding to the attribute separately in a extension scheme. That way, the content provider can substitute a binding to a different extension without rework.

An adopter would identify the extension scheme as the scheme governing controlled values in the DITA environment. Any base schemes referenced by the extension scheme are, from a logical view, part of the extension scheme.

Attributes

Name	Description	Data Type	Default Value	Required?
navtitle	Specifies the title of the topic as it will appear in the navigation or tables of contents that are generated from the map. Beginning with DITA 1.2, the preferred way to specify the navigation title in a map is with the navtitle element, available inside the topicmeta element.	CDATA	#IMPLIED	No
href	A pointer to the resource represented by the <topicref>. See The href attribute for detailed information on supported values and processing implications. References to DITA content cannot be below the topic level: that is, you cannot reference individual elements inside a topic. References to content other than DITA topics should use the format attribute to identify the kind of resource being referenced.	CDATA	#IMPLIED	No
keys	Introduces one or more global identifiers for a resource referenced from a map. See The keys attribute for details on how to use the keys attribute.	NMTOKEN	#IMPLIED	No
processing-role	Describes the processing role of the referenced topic. The processing default is "normal". If the value is not specified locally, but is specified on an ancestor, the value will cascade from the closest ancestor. normal Normal topic that is a readable part of the information. resource-only The topic is used as a resource for processing purposes, but is not a readable unit of information on its own. This topic should not be included in a rendered table of contents, and the topic should not be rendered on its own. -dita-use-conref-target See Using the -dita-use-conref-target value for more information.	(normal \| resource-only \| -dita-use-conref-target)	#IMPLIED	No
query	This attribute is deprecated. It may be removed in the future.	CDATA	#IMPLIED	No
type	Describes the target of a reference. For the schemeref element, this value defaults to "scheme", because the element is expected to point to another subject scheme.	CDATA	scheme	No
format	The format attribute identifies the format of the resource being referenced. For elements like this one that are designed to reference another map, the value defaults to "ditamap". See The format attribute for details on other supported values.	CDATA	ditamap	No
scope	The scope attribute identifies the closeness of the relationship between the current document and the target resource. See The scope attribute for more information on values.	(local \| peer \| external \| -dita-use-conref-target)	#IMPLIED	No
univ-atts attribute group (includes select-atts, id-atts, and localization-atts groups)	A set of related attributes, described in univ-atts attribute group
global-atts attribute group (xtrf, xtrc)	A set of related attributes, described in global-atts attribute group
class, outputclass, keyref	Keyref provides a redirectable reference based on a key defined within a map. See The keyref attribute for information on using this attribute. Class and outputclass are described in Other common DITA attributes