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Re: pair interactions in XML schema

From: Jeni Tennison <jeni@jenitennison.com>
Date: Wed, 19 Jun 2002 10:49:13 +0100
Message-ID: <120-1635183550.20020619104913@jenitennison.com>
To: Jos van den Oever <oever@fenk.wau.nl>
CC: xmlschema-dev@w3.org

Hi Jos,

> I think I see now. So each object can have only one key of a certain
> kind. But a list of keys can contain different types of objects.

I think you're using 'key' in a different way from the way I do, but
yes. Within each identity constraint, each element has a key value
associated with it. That key value comes from combining the fields
defined in the identity constraint, and for each element in the
instance document, each field must only pick one element or attribute.

> So this is valid:
>
>>   <xs:key name="people">
>>     <xs:selector xpath="person1 | person1/person2 |
>>                         person1/person2/person3" />
>>     <xs:field xpath="@name" />
>>   </xs:key>
>
> But this isn't:
>  <xs:key name='acquaintances'>
>    <xs:selector xpath='acquaintance' />
>    <xs:field xpath='@person1|@person2' />
>  </xs:key>

That *could* be valid -- it depends on the source XML. If you had XML
like:

  <acquaintance person1="John" />
  <acquaintance person2="Fred" />

i.e. where each acquaintance element could only have one of either
person1 or person2 attributes, then the identity constraint is fine.
On the other hand, if you had an acquaintance element like:

  <acquaintance person1="John" person2="Fred" />

then that wouldn't be valid, because the field's XPath would select
two nodes. (This shows how you can use identity constraints to test
some co-occurrence constraints in XML Schema, BTW.)

Note that the acquaintance with both person1 and person2 attributes is
valid with:

  <xs:key name="acquaintances">
    <xs:selector xpath="acquaintance" />
    <xs:field xpath="@person1" />
    <xs:field xpath="@person2" />
  </xs:key>

but the acquaintance elements with only one and not the other are not
valid. You could make them valid as well if you use xs:unique rather
than xs:key in the above (xs:key means that the field *must* select
one node, whereas xs:unique means that the field might not find a
node).

> I'm using xerces to validate xml files. However, xerces does not
> seem to agree with these rules. It validates this:
>
>   <person1 name='Pete'>
>    <person2 name='Pete'/>
>   </person1>

What does the identity constraint look like?

> and also, it does not give an error message if I use a scheme file
> with a field like this:
>  <xs:field xpath='@person1|@person2' />

Having this in a schema isn't an error with the schema (XPaths are too
complicated to validate, so the only kinds of errors in them that are
detectable are those that occur in the XPath syntax, like trying to
use a predicate for example). It should be an error if you have a
selected element in the instance document that has both a person1 and
a person2 attribute.

Cheers,

Jeni

---
Jeni Tennison
http://www.jenitennison.com/
Received on Wednesday, 19 June 2002 05:49:15 GMT

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