XSD: Extending Code Lists with xsd:union

In certain circumstances it is necessary to add elements to an existing NIEM enumeration (or code list).  In these situations one may choose to simply recreate a new list with all the same elements already defined in a NIEM code type and simply add those which do not yet exist.  However, when the code list is larger than a few elements (such as a state code list with at least 50 valid values), using xsd:union as an option becomes more appealing.

The xsd:union provides a way to combine simple data types together to form a larger and more comprehensive data type.  An example would be simply adding “ZZ” to a list of US Postal Service State (USPS) Codes to communicate an unknown or invalid state.  This can be accomplished by extending the existing USPS code list in several steps.

Step 1 – Create a New Simple Type With New Values

<!-- Simple code value to add ZZ as a valid value -->
<xsd:simpletype name="USStateCodeDefaultSimpleType">
  <xsd:restriction base="xsd:token">
   <xsd:enumeration value="ZZ">
    <xsd:annotation>
     <xsd:documentation>UNKNOWN</xsd:documentation>
    </xsd:annotation>
   </xsd:enumeration>
  </xsd:restriction>
</xsd:simpletype>

Step 2 – Use xsd:union to Join the Values with Existing Values

<!-- New simple time combining my custom enum with the standard usps one --> 
<xsd:simpleType name="LocationStateCodeSimpleType">
  <xsd:union memberTypes="usps:USStateCodeSimpleType my:USStateCodeDefaultSimpleType"/>
</xsd:simpleType>

Step 3 – Wrap the New Simple Data Type in a Complex Type

<!-- New complexType required to add s:id and s:idref to the definition -->
<xsd:complexType name="LocationStateCodeType">
  <xsd:simpleContent>
    <xsd:extension base="aoc_code:LocationStateCodeSimpleType"> 
      <xsd:attributeGroup ref="s:SimpleObjectAttributeGroup"/>
    </xsd:extension>
  </xsd:simpleContent>
</xsd:complexType>

Step 4 – Create Element Instantiating the New Code List

<!-- Element declaration allowing use of our new data type -->
<xsd:element name="NewStateCode" type="my:LocationStateCodeType" substitutionGroup="nc:LocationStateCode"/>

Now any place an nc:LocationStateCode can be use, our extended code list can be used instead.