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Re: XML Schema: At Least One of X Elements Must Appear

• To: <xml-dev@l...>
• Subject: Re: XML Schema: At Least One of X Elements Must Appear
• From: "Rick Jelliffe" <ricko@a...>
• Date: Mon, 19 Aug 2002 03:52:20 +1000
• References: <3A967FE7B99E134C9FE65E2C136560300DE932@l...>

From: "CHIUSANO, Joseph" <JCHIUSANO@l...>

> I am 99.9% sure that there is no way to represent in XML Schema that "at
> least one of the following X elements must appear in the XML document" -
> ... In other words, I don't believe that with the above approach (and
> using XML Schema) I can satisfy the requirement that "at least one of the
> possible REF01 qualifier values must be represented in the transaction".  I
> want to be 100% sure before I tell a client this.

In general, W3C XML Schemas 1.0 (WXS) cannot do these kind of constraints.

In your particular case, there may be an approach using keys, though I may
be hallucinating.

You add a default (fixed) declaration to some convenient elemet (e.g. the
root) which annotates the document with an attribute to the effect of
  required-REF01="FS"

Then you declare that attribute is a keyref. Then you declare that the
REF01 attribute is a key, but not unique.  

Now when you validate, the validator will check that there is an element
with REF01="FS" somewhere to match your required-REF01 attribute.

Sorry I have no code for this; I may be wrong.  In passing, even if WXS can
handle this case (which would be nice), it still may be prudent to adopt
Schematron as a standard part of your architecture. That way whenever
you have issues like this, you are not left scratching your head, but you
can rapidly get your constraints implemented.   {As a matter of software
engineering, how can you test your Schema (and implementation) 
works? You run enough cases through and evaluate them: and you need
some tool that is not based on XML Schemas to do the evaluation.  }

Cheers
Rick Jelliffe

Cheers
Rick

• Follow-Ups:
  • Re: XML Schema: At Least One of X Elements Must Appear
    • From: ht@c... (Henry S. Thompson)

• References:
  • XML Schema: At Least One of X Elements Must Appear
    • From: "CHIUSANO, Joseph" <JCHIUSANO@l...>

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