Re: Question on id constraints

[Eddie Robertsson <eddie@allette.com.au>]

Hi John,

Let me see if I can shed some light on the key, unique and keyref issue.

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)

No, the key declaration doesn't need to be declared physically before the
keyref declaration. The rule is that the scoping element for the key (or
unique) declaration must be the same as OR a descendant of the scoping
element for the keyref declaration.
This means that the following is correct (not complete XML Schema syntax!):

<xs:element name="root">

   <xs:element name="sub" type="whatever">
      <xs:key name="key">
         <xs:selector xpath="key"/>
         <xs:field xpath="@id"/>
      </xs:key>
   </xs:element>

   <xs:keyref name="keyref" refer="key">
      <xs:selector xpath="keyref"/>
      <xs:field xpath="@ref"/>
   </xs:keyref>

</xs:element>

The scoping element for the key declaration is a descendant of the scoping element for the keyref declaration.

However this is not allowed:

<xs:element name="root">

   <xs:element name="sub" type="whatever">
      <xs:keyref name="keyref" refer="key">
         <xs:selector xpath="keyref"/>
         <xs:field xpath="@ref"/>
      </xs:keyref>
   </xs:element>

   <xs:key name="key">
      <xs:selector xpath="key"/>
      <xs:field xpath="@id"/>
   </xs:key>

</xs:element>

> 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?
The same as OR a descendant.

Cheers,
/Eddie