Re: Multiple key/key symbol spaces

[James Clark <jjc@jclark.com>]

> What I don't understand now is how you refer to scoped keys.
>
> If the department is unique only within a company, can it be referenced?
> Or Isn't it something to be referenced?

This is the big problem with scoped keys.  XML Schema cannot handle
hierarchical references well (at all really).

Let's take a concrete example.

<element name="vehicleRegistrationDatabase">
  <element name="vehicles>
     <zeroOrMore>
        <element name="state">
           <attribute name="stateCode"/>
           <zeroOrMore>
              <element name="vehicle">
                 <element name="owner.ssn"><anyString/></element>
                 <element name="regno"><anyString/></element>
               </element>
            </zeroOrMore>
          </element>
       </zeroOrMore>
    </element>
    <element name="persons">
        <zeroOrMore>
           <element name="person">
             <attribute name="ssn"/>
              <zeroOrMore>
                 <element name="ownedVehicle">
                     <attribute name="stateCode"/>
                     <attribute name="regno"/>
                  </element>
              </zeroOrMore>
            </element>
        </zeroOrMore>
     </element>
</element>

For such a schema, we might have a hierarchical IDREF constraint as follows.
So for every vehicleRegistrationDatabase $x, for every
$x/persons/person/ownedVehicle $v, there must be a $x/vehicles/state $s such
that

- $s/@stateCode = $v/@stateCode and
- there is a $s/vehicle $w such that $w/regno = $v/@regno.

One could imagine encoding this by modifying the pattern for
vehicleRegistrationDatabase along the following lines:

<element name="vehicleRegistrationDatabase">
  <keyRef>
    <selector>persons/person/ownedVehicle</selector>
    <ref>
        <from>@stateCode</from>
        <to>
             <selector>vehicles/state</selector>
              <field>@stateCode</field>
              <ref>
                   <from>@regno</from>
                   <to>
                       <selector>vehicle</selector>
                       <field>regno</field>
                   </to>
              </ref>
         </to>
      </ref>
  </keyRef>
  <element name="vehicles">
     ...
</element>

James