Schematron: License Plate State is Required when a Number Exists

A common practice in transportation and law enforcement is to document a vehicle’s license plate number.  In many situations, this plate number must be accompanied by the state which issued the license plate. 

In NIEM, a vehicle’s license plate is contained within the nc:ConveyanceRegistrationPlateIdentification element which is an nc:IdentificationType.  Using schema cardinality, one could make a the state required by simply assigning a minOccurs=”1” to the nc:IdentificationJurisdiction element, however this can often cause more problems than it solves for two key reasons:

1. Making jurisdiction required through schema cardinality makes it required globally throughout the exchange even if it doesn’t apply in those scenarios as many other elements in a typical NIEM exchange are also nc:IdentificationType data types.
2. nc:IdentificationJurisdiction is an abstract data element that can be replaced with any number of elements, not all of which are enumerated state values.  Some are country codes, some are province codes for other countries and others are simply free-text. 

This presents another ideal use case for Schematron.  The following example code segment ensures a NCIC plate issuing state is included any time a Plate Identification exists:

<pattern id="eVehiclePlateState">
  <title>Ensure a plate state is included with a plate number.</title>
  <rule context="ns:MyDocument/nc:Vehicle/nc:ConveyanceRegistrationPlateIdentification">
    <assert test="j:IdentificationJurisdictionNCICLISCode">
      A plate state must be included with vehicle license plate.
    </assert>
  </rule>
</pattern>

The same segment can be modified to enforce any of the available jurisdiction code lists.  For example, an exchange in Canada may wish to check for the existence of j:IdentificationJurisdictionCanadianProvinceCode instead of j:IdentificationJurisdictionNCICLISCode.