How to prevent self-referencing foreign key values in XSD schema? - xsd

Question

My XML (simplified) is like this:
<Actions>
<Action Id="1">
</Action>
<Action Id="2">
<DoSomething>
<ActionRef ActionId="1" /> <!-- valid -->
</DoSomething>
</Action>
</Actions>
The ActionId attribute value references the Id attribute value of the Action element. I've already set up a foreign key constraint in the XSD, and it works correctly.
I want to prevent self-referencing values in the foreign field, like this:
<Actions>
<Action Id="1">
</Action>
<Action Id="2">
<DoSomething>
<ActionRef ActionId="2" /> <!-- invalid -->
</DoSomething>
</Action>
</Actions>
Of course, this can easily be done within the application that processes the XML, and I'll fall back on that if what I'm asking for isn't possible, but I'd much rather have this done automatically by the validation process.
I tried adding [not(#ActionId = ../#Id)] to the foreign key selector XPath query, but that isn't valid in that context (nor am I sure it's correct either). Other than that, I have no idea what else to try, and it doesn't look like many people on the internets even set up foreign key relationships in their XSDs, let alone prevent this kind of situation (I found nothing on this exact topic).

Answer 1

It cannot be done - the selector syntax for XSD constraints is very limited. Other alternatives may include Schematron, which should be reasonable to integrate assuming your runtime has access to an XSLT processor. The effort could pay off is you decide to add more validation rules separate from the code of the application that processes the XML.