Question

I’m trying to make two XML attributes to be mutually exclusive. How can one create an XSD schema to capture this kind of scenario?

I would like to have one of these

<elem value='1' /> <elem ref='something else' /> 

but not

<elem value='1' ref='something else' /> 

Answer 1

Since RelaxNG was mentioned in Alnitak’s answer, here is a solution with RelaxNG (a language which is, in most cases, better than W3C Schema). Do note the OR (|) in the definition of elem:

start = document document = element document {elem+} elem = element elem {ref | value} ref = attribute ref {text} value = attribute value {xsd:integer} 

If I have this XML file:

<document>     <elem value='1' />     <elem ref='something else' /> </document> 

It is accepted by rnv and xmlint:

 % rnv attributes-exclusive.rnc attributes-exclusive.xml               attributes-exclusive.xml   % xmllint --noout --relaxng attributes-exclusive.rng attributes-exclusive.xml   attributes-exclusive.xml validates 

If I add in the XML file:

<elem value='1' ref='something else' /> 

I get validation errors, as I want (do note that the error messages are suboptimal):

% rnv attributes-exclusive.rnc attributes-exclusive.xml     attributes-exclusive.xml attributes-exclusive.xml:4:0: error: attribute ^ref not allowed required:        after  % xmllint --noout --relaxng attributes-exclusive.rng attributes-exclusive.xml attributes-exclusive.xml:4: element elem: Relax-NG validity error : Invalid attribute value for element elem attributes-exclusive.xml fails to validate