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Re: How to do a choice on aggregation with w3C XML schema

• To: santony@b...
• Subject: Re: How to do a choice on aggregation with w3C XML schema
• From: John Cowan <cowan@m...>
• Date: Thu, 18 Dec 2003 09:14:35 -0500
• Cc: xml-dev@l...
• In-reply-to: <20031218053416.QAKB17885.imf20aec.mail.bellsouth.net@m...>
• References: <20031218053416.QAKB17885.imf20aec.mail.bellsouth.net@m...>
• User-agent: Mutt/1.3.28i

santony@b... scripsit:

> We need to essentially build a schema that will restrict the xml to
> be in one of these 3 forms. Is it possible to do this at all.

Not in W3C XML Schema.  So if you need this kind of validation, get
with the program and use RELAX NG, which is perfect for this kind of thing.
Here's a (partial) RELAX NG schema, in compact notation, that will
validate your documents:

start = command1 | command2 | command3

command1 = element COMMAND "REQUEST1",
	   element COMMAND_DATA {
		element REQ1_DAT1 {text},
		element REQ1_DAT2 {text},
		...
	   }

command2 = element COMMAND "REQUEST2",
	   element COMMAND_DATA {
		element REQ2_DAT1 {text},
		element REQ2_DAT2 {text},
		...
	   }

command3 = element COMMAND "REQUEST3",
	   element COMMAND_DATA {
		element REQ3_DAT1 {text},
		element REQ3_DAT2 {text},
		...
	   }

Now this assumes that you really do know how many REQi_DATj elements
exist for each type; no extant schema language can help you much if you
are allowed an arbitrary number of them.  The "..." is not part of RELAX
NG syntax any more than it is of XML Schema.

> But though my xml editor ( xmlspy ) reported this to be a valid schema, 

XML Spy is notoriously unreliable at checking what is and what is not a
valid XML Schema.  Indeed, no schema should be called valid unless it's
been checked with several different validators.

> When I used xerces to validate I got an error like
> 
> "http://www.bellsouth.com/wfaif":COMMAND and
> "http://www.bellsouth.com/wfaif":COMMAND (or elements from their
> substitution group) violate "Unique Particle Attribution". During
> validation against this schema, ambiguity would be created for those
> two particles"

To XML Schema, your choice looks like "COMMAND | COMMAND | COMMAND", and
it can't cope.  Differentiating on the content of the element is too subtle.
RELAX NG, on the other hand, eats this stuff up.

You can find RELAX NG validators and other tools for various environments
at http://www.relaxng.org .

Moral:  Don't define document types like this.  Let ordering do its
natural work next time, instead of doubling up with ordering *and*
magic element names.

-- 
"May the hair on your toes never fall out!"     John Cowan
        --Thorin Oakenshield (to Bilbo)         jcowan@r...

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