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Re: Question on id constraints

1.  It is not necessary for the pk to be defined earlier in the schema file according to an example from the W3C.  In the example that they give they create a keyref that refers to another labeled "state" which doesn't occur until much later.

<xs:keyref name="vehiclestate" refer="state">
    <xs:selector xpath=".//vehicle"/>
    <xs:field xpath="@state"/>


<xs:element name="root">
       <xs:element ref="state" maxOccurs="unbounded"/>

   <xs:key name="state">
      <xs:selector xpath=".//state"/>
      <xs:field xpath="code"/>

I would assume that the reasoning for this is that the key must be defined within context that it is in and not within a special order.  If there is a key with respect to an element then it must be defined with in that element definition.  If the ordering mattered then that would limit the potency of xml.  Say for example a element has a key reference to another key within another element and that element has a key reference to a key within the first element, then there would be no way to define this.

2.  No it is not required for pk to be defined within the same element as the keyref.  If you think of the element definition as a database table and the pk representing a primary key, then it wouldn't make sense for you to define a key reference to that key within the same table.  Instead the more likely use of key references is to refer to a key within another table (or element within this case).

I hope that this information helped,
Laura Hatcher

"Anderson, John" wrote:



I'm trying to decipher the schema spec on identity contstraint validity, but perhaps someone can save me a few hours and a few aspirin.

If I have:
<keyref name="ref" refers="pk" > ...

1. Is the key "pk" required to be defined physically earlier in the schema file (I would have thought not)
2. Is pk required to be defined on the same element as ref, or on a descendant or anscetor of it, or just anywhere in the same schema?



John Anderson
CTO BarbadosoftTM  Pure intelligent XML
+31 (0)20 750 7582 / +31 (0)6 65 347 448 / www.barbadosoft.com

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