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   RE: Standard way to express object, not resource, rels in XML?

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  • From: Robert Worden <rworden@dial.pipex.com>
  • To: 'Mike Brown' <mbrown@corp.webb.net>,"'xml-dev@lists.xml.org'" <xml-dev@lists.xml.org>
  • Date: Sun, 20 Aug 2000 22:03:34 +0100

Mike Brown [SMTP:mbrown@corp.webb.net] wrote:
> I want to use XML to express relationships between instances of the
> conceptual/abstract objects that are each modeled by separate XML 
> (or separate collections of documents).

Do you always have a separate XML document to model each object? Isn't it 
more common to have several different object classes in your model, and 
typically many object instances per document, with relationships both 
within and across documents?

Expressing relationships (UML: associations) between objects even within 
one XML document is not that simple, because XML gives you so many ways to 
do it.  XML can express a relationship between two object instances in any 
of the following ways:

- as an idref/id pair
- by shared values of attributes or elements (just as relational databases 
express relations by 'foreign keys')
- by nesting of elements (element representing object A inside element 
representing object B)
- by 'denormalisation', attaching the attributes of both objects to the 
same element
- separately, outside the elements representing each object

There are many variants on these and they are not just academic; you can 
find all of them in commonly used XML languages. So there are lots of 
'standard' ways to express object relations; although I believe initiatives 
like ebXML would like to encourage some rather than others (shutting the 
stable door after the horse has bolted...?). So did the BizTalk 'canonical 
form', before BizTalk started accepting everybody's schemas.

This plethora of ways to express relations is what makes XML translation 
hard - preserving the relationships while translating them from one 
representation to another. We have built a tool that can understand and 
translate all the known relation representations within a document, but not 
yet relations across documents.

Robert Worden



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