tests/idc/jira_1594.xsd

<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">

  <xs:element name="personnel">
    <xs:complexType>
      <xs:sequence>
        <xs:element ref="person" maxOccurs="unbounded"/>
      </xs:sequence>
    </xs:complexType>
    <xs:key name="key">
      <xs:selector xpath="person"/>
      <xs:field xpath="@id"/>
    </xs:key>
    <xs:keyref refer="key" name="keyref">
      <xs:selector xpath="person"/>
      <xs:field xpath="ref"/>
    </xs:keyref>
  </xs:element>

  <xs:element name="person">
    <xs:complexType>
      <xs:sequence>
        <xs:element name="name" type="xs:string"/>
        <xs:element name="ref" type="xs:integer" minOccurs="0"/>
      </xs:sequence>
      <xs:attribute name="id" type="xs:integer" use="required"/>
    </xs:complexType>
  </xs:element>

</xs:schema>