Is it possible to restrict an IDREF attribute so that it only matches child ID values?

Question

Suppose I have the following XML snippet:

<x child="value">
  <y id=z/>
</x>

I want to ensure that "x/@child" is limited, in this case, to "z".  ("value" is a placeholder.")  How do I do that?

---

Will Pittenger

Answer 1

I saw http://stackoverflow.com/questions/891324/xsd-key-keyref-hierarchical-key-structure.  In there they talked about this sample of code (translated to use the above names):

<xs:element name="x">
  <xs:complexType>
    <xs:sequence>
      <xs:element name="y">
        <xs:complexType>
          <xs:attribute name="id" type="NAME"/>
        </xs:complexType>
      </xs:element>
    </xs:sequence>
    <xs:attribute name="child" type="NAME"/>
    <xs:key name="child-name">
      <xs:selector xpath="x/y"/>
      <xs:field xpath="../@id"/>
      <xs:field xpath="@id"/>
    </xs:key>
  </xs:complexType>
</xs:element>

They noted how ".." isn't allowed in field xpaths, which I verified.  But in a flash of what I hope was insight, I changed the xpaths.  Compare this code with what is above:

<xs:element name="x">
  <xs:complexType>
    <xs:sequence>
      <xs:element name="y">
        <xs:complexType>
          <xs:attribute name="id" type="NAME"/>
        </xs:complexType>
      </xs:element>
    </xs:sequence>
    <xs:attribute name="child" type="NAME"/>
    <xs:key name="child-name">
      <xs:selector xpath="x"/>
      <xs:field xpath="@id"/>
      <xs:field xpath="y/@id"/>
    </xs:key>
  </xs:complexType>
</xs:element>

The question now is, will that cause the XSD to do what I want?  I am concerned the "y/@id" will only take one child per parent.  Next, do I put the <xs:keyref> inside the <xs:attribute> element?  Or somewhere else?

---

Will Pittenger