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Re: XML schema limitation?

From: Henry S. Thompson <ht@cogsci.ed.ac.uk>
Date: 22 Aug 2001 13:27:03 +0100
To: "Stephane Pageau" <spageau@sed.ara.com>
Cc: <xmlschema-dev@w3.org>, <jpowell@sed.ara.com>
Message-ID: <f5bvgjgp9ew.fsf@cogsci.ed.ac.uk>
"Stephane Pageau" <spageau@sed.ara.com> writes:

> How can I construct a XML schema in which 
> I wish to enforce that the content 
> of an element (e.g., a subelement) 
> be an existing element of the instance
> XML file?
> To clarify this question let me take an example:
> ------------------------------------------------
> Assume that I have a shape described using a 
> bunch of vertices and I describe both the
> vertices and the shape separately as follows: 
> <vertex id=10 > 
>    <x> 10. </x>
>    <y> 60. </y>
> </vertex>
> <vertex id=20 > 
>    <x> 10. </x>
>    <y> 45. </y>
> </vertex>
> <vertex id=30 > 
>    <x> 10. </x>
>    <y> 50. </y>
> </vertex>
> ...
> <shape id=1>
>    <vertexptr> 10 </vertexptr>
>    <vertexptr> 30 </vertexptr>
>    <vertexptr> 20 </vertexptr>
> </shape>
> How can I construct my schema to make sure that someone does not
> put <vertexptr> 1000 </vertexptr> if
> the vertex with id=1000 does not exist?

Sure -- make the content of <vertexptr> be of type IDREF.

As the REC notes, this is not backward compatible with DTDs -- if you
changed to <vertexptr ref="30"/> that would no longer be a problem.

Oops -- you can't use integers as IDs in XML.

So make vertex ids be keys, and the content/ref of vertexptr be
keyrefs.  See [1] in the REC or [2] in the Primer.

ht

[1] http://www.w3.org/TR/xmlschema-1/#cIdentity-constraint_Definitions
[2] http://www.w3.org/TR/xmlschema-0/#specifyingUniqueness
-- 
  Henry S. Thompson, HCRC Language Technology Group, University of Edinburgh
          W3C Fellow 1999--2001, part-time member of W3C Team
     2 Buccleuch Place, Edinburgh EH8 9LW, SCOTLAND -- (44) 131 650-4440
	    Fax: (44) 131 650-4587, e-mail: ht@cogsci.ed.ac.uk
		     URL: http://www.ltg.ed.ac.uk/~ht/
Received on Wednesday, 22 August 2001 08:26:41 GMT

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