xsd:unique with optional attributes

Question

I have this Xml file:

<objects>
  <object name="ID1" />
  <object name="ID2" />
  <object name="ID2" color="green" />
  <object name="ID3" color="green" />
<objects>

I would like to validate this against an XSD Schema, so that the combination between name and color are unique in the document.

The problem is that, if I use:

<xs:unique name="UniqueObjectNameColor">
  <xs:selector xpath="./object" />
  <xs:field xpath="@name" />
  <xs:field xpath="@color" />
</xs:unique>

... the rule will ignore object elements without the optional color attribute. The following validates correctly while it shouldn't.

  <object name="ID2" />
  <object name="ID2" />

Can you tell me how can I specify a rule that enforces unique name and color combinations and, when the color attribute is not present in the element object, it checks the name?

Answer 1

Use use and default with or without a value like:

    <element name="objects">
        <complexType>
            <sequence>
                <element name="object" maxOccurs="unbounded">
                    <complexType>
                        <attribute name="name" type="string" />
                        <attribute name="color" type="string" use="optional" default="noColor" />
                    </complexType>
                </element>
            </sequence>
        </complexType>
        <unique name="UniqueObjectNameColor">
            <selector xpath="tns:object" />
            <field xpath="@name" />
            <field xpath="@color" />
        </unique>
    </element>

</schema>