Question

I’m not sure if this is possible with XML/XSD, and haven’t had much luck searching for it.

Can you specify an XSD, so that when you’re writing your XML, you can have an element that must reference one of the values contained in another XML element?

For example, if you had this XML:

<car>Audi</car>
<car>Ford</car>
<car>Honda</car>

<person>
  <drives>Audi</drives>
</person>

How do you specify the XSD for drives so that it must be one of the values entered for car?

Answer 1

With XSD 1.1 you can use assertions.

Assertion components constrain the existence and values of related
elements and attributes.

You can use XPath 2.0 expressions to perform the validation/test in the assertion.

This example schema has an assertion for the /doc element that ensures that the /doc/person/drives value is equal to one of it’s /doc/car elements:

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" version="1.1" elementFormDefault="qualified">

    <xs:element name="car" type="xs:string" />  
    <xs:element name="drives" type="xs:string" />

    <xs:element name="person">
        <xs:complexType>
            <xs:sequence>
                <xs:element ref="drives"/>
            </xs:sequence>     
        </xs:complexType>
    </xs:element>

    <xs:element name="doc">
        <xs:complexType>
            <xs:sequence>
                <xs:element ref="car" maxOccurs="unbounded"/> 
                <xs:element ref="person"/>
            </xs:sequence> 
            <xs:assert test="person/drives = car"/>
        </xs:complexType>
    </xs:element>

</xs:schema>