Implementing XOR on an Enumeration List - xsd

Question

The best way to illustrate my question is by posting some snippets of the xsd I currently have; it'll make it easier for you to make sense of my question.
Here's some snippets of my current xsd file:
<xs:element name="RiskAnalysis">
<xs:complexType>
<xs:sequence>
<xs:element ref="RiskRating" maxOccurs="unbounded"/>
</xs:sequence>
...
...
</xs:complexType>
</xs:element>
<xs:element name="RiskRating">
<xs:complexType>
...
<xs:attribute name="RatingType" use="required">
<xs:simpleType>
<xs:restriction base="xs:NMTOKEN">
<xs:enumeration value="LocationNeighbourhood"/>
<xs:enumeration value="Land"/>
<xs:enumeration value="Improvements"/>
<xs:enumeration value="Environmental"/>
<xs:enumeration value="MarketSegment"/>
<xs:enumeration value="ReducedValue"/>
<xs:enumeration value="RecentMarket"/>
<xs:enumeration value="LocalEconomy"/>
<xs:enumeration value="MarketVolatility"/>
<xs:enumeration value="Other"/>
</xs:restriction>
</xs:simpleType>
</xs:attribute>
...
</xs:complexType>
</xs:element>
The issue I'm having is that I need to restrict my xsd such that I create an XOR for the RiskRatings between elements where RatingType is ReducedValue and RecentMarketValue, but I can't think of a way to do this.
It's best described by example, thus here's some sample RiskAnalysis elements (in xsd pseudo code) I might receive:
Acceptable (ie. neither of ReducedValue or RecentMarket passed in):
<RiskAnalysis>
<RiskRating RatingType="Land" />
<RiskRating RatingType="Other" />
</RiskAnalysis>
Acceptable (ie. only ReducedValue passed in):
<RiskAnalysis>
<RiskRating RatingType="Land" />
<RiskRating RatingType="Other" />
<RiskRating RatingType="ReducedValue" />
</RiskAnalysis>
Acceptable (ie. only RecentMarket passed in):
<RiskAnalysis>
<RiskRating RatingType="Land" />
<RiskRating RatingType="Other" />
<RiskRating RatingType="RecentMarket" />
</RiskAnalysis>
Unacceptaple and should throw a schema validation error (ie. both RecentMarket and ReducedValue passed in):
<RiskAnalysis>
<RiskRating RatingType="Land" />
<RiskRating RatingType="Other" />
<RiskRating RatingType="RecentMarket" />
<RiskRating RatingType="ReducedValue" />
</RiskAnalysis>
Anybody got any idea how I would do this?

Answer 1

Its not possible with XSD (alone)!!
workaround solution: You many need to use HOST CODE to evaluate this part only. The host program which is invoking XSD validation like C#, CPP, JAVA, etc ... are capable of validating these kind of conditions.

Answer 2

The simplest way would be to eliminate the inconsistency in the analysis of your domain implicit in your schema: on the one hand, you want risk ratings of type RecentMarket and of type ReducedValue to be treated differently for purposes of validation, while on the other hand you are giving them the same element type, which says implicitly that for purposes validation they should be treated the same. The same? or different? Choose.
Given element types RecentMarketRating, ReducedValueRating, and OtherRiskRating, it's trivial to solve your problem. Given an XML encoding which calls them all the same thing, you main options are non-XSD code to do the validation, XSD 1.1 and the use of conditional type assignment or assertions, or Schematron in addition to your XSD 1.0.