XPath on xs:selector for a xs:keyref

Thank you in advance for any help you can give...

I have a problem. What I'd like to do is define a relationship in an XSD between two tables, where the relationship returns the last row in the child table by default. I thought that the following should work:

<xs:keyref name="project_taskproject_eventlast" refer="project_taskkey1" msdata:DeleteRule="Cascade" msdata:UpdateRule="Cascade">
<xs:selector xpath=".//project_event[last()]"/>
<xs:field xpath="proj_task_id"/>
</xs:keyref>

The problem is that the .Net compiler for this returns an error saying that the xpath found in the selector is invalid.

Now, I tried helping myself by plowing through the W3C specifications to see if this was even allowed, and though I found references to the xpath being allowed for a selector only being a subset of all of valid xpath, I couldn't find a specific reference to what was, and was not allowed.

Is there anyway for me to do this from the XSD itself Thanks again!

.NET Development31

Hi Andy,

Xml Schema indeed only allows a subset of the XPath to be used in the field and selector elements. The formal definition can be found here: http://www.w3.org/TR/xmlschema-1/#coss-identity-constraint

Stan Kitsis

I tried to understand the specifications but sometimes is not easy.
I cannot find where it is defined the fact the XPath selector expression cannot be something like:

selector=".//generic-element"

I think .//element-name is possible but the original poster in this thread had a predicate behind that (e.g. [last()]) and while XPath in general allows predicates to filter out nodes the XPath subset defined for the XSD schema language does not allow predicates.

I would like to continue this thread because I really need to understand the specification of key/keyrefs and their selectors.

I need to create in some way a recursive mechanism. I am posting you a short schema example:

< xml version="1.0" encoding="UTF-8" >

<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" elementFormDefault="qualified" attributeFormDefault="unqualified">
<xs:element name="Book" type="Book_CT">
<xs:key name="ChapterKey">
<xs:selector xpath="Chapter"/>
<xs:field xpath "/>
</xs:key>
<xs:keyref name="ChapterKeyRef" refer="ChapterKey">
<xs:selector xpath="TOC"/>
<xs:field xpath="Chapter"/>
</xs:keyref>
</xs:element>
<xs:complexType name="Book_CT">
<xs:sequence>
<xs:element name="Title" type="xs:string"/>
<xs:element name="Author" type="xs:string"/>
<xs:element name="TOC" type="TOC_CT"/>
<xs:element name="Chapter" type="Chapter_CT"/>
</xs:sequence>
</xs:complexType>
<xs:complexType name="Chapter_CT">
<xs:sequence minOccurs="0">
<xs:element name="Chapter" type="Chapter_CT" minOccurs="0" maxOccurs="unbounded"/>
</xs:sequence>
<xs:attribute name="Name" type="xs:string" use="required"/>
</xs:complexType>
<xs:complexType name="TOC_CT">
<xs:sequence>
<xs:element name="Chapter" type="xs:string"/>
</xs:sequence>
</xs:complexType>
</xs:schema>

And the correspondent XML project

<Book xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="..\..\Desktop\Untitled1.xsd">
<Title/>
<Author/>
<TOC>
<Chapter>Chapter 1</Chapter>
</TOC>
<Chapter Name="Chapter 1">
    <Chapter Name="Chapter 2.1"/>
    <Chapter Name="Chapter 2.2">
        <Chapter Name="Chapter 2.2.1"/>
     </Chapter>
</Chapter>
</Book>

This example works, but not in the way I like.

The problem is that the definition

<xs:key name="ChapterKey">

<xs:selector xpath="//Chapter"/>

<xs:field xpath "/>

</xs:key>

is not accepted by Altova. Is a problem of Altova, or the XML Schema specification do not allow this