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potential erratum in XML Schema Primer

From: <neilg@ca.ibm.com>
Date: Fri, 22 Jun 2001 17:43:55 -0400
To: www-xml-schema-comments@w3.org
Message-ID: <OF39EAD0F9.3DAF277A-ON85256A73.00745044@torolab.ibm.com>
Hello,

From 3.11.4 of the Schema Structures specification we have:

Validation Rule: Identity-constraint Satisfied

4.3 If the
{identity-constraint category}
is keyref, then for each member of the
·qualified node set· (
call this the keyref member), there must be a
·node table·
associated with the
{referenced key}
in the
[identity-constraint table]
of the element information item (see
Identity-constraint Table (§3.11.5),
which must be understood as logically prior to this clause of this
constraint, below) and there must be an entry in that table whose
·key-sequence·
is equal to the keyref member's
·key-sequence·
member for member, as defined by
Equal
in
[XML Schemas: Datatypes].

In lay terms, I take this to mean that:

1.  For all sequences of strings identified by the fields of a <keyref>,
there must exist a sequence of pairwise equal strings identified by the
identity constraint that the <keyref> references;

2. Let element K be an element in the instance document which is validated
by a schema declaration which contains a <keyref>.  Now consider the set of
nodes S  identified by applying the xpath descendant-or-self axis to K.
Then one of the nodes in S must be validated by an element declaration
containing an identity constraint which the "refer" attribute of the
<keyref> resolves to.  That is to say, the identity constraint referenced
by the <keyref> must apply to the element the <keyref> is on or to one of
its descendants in a given instance document.

If the above is correct, then the following example from the beginning of
chapter 5 of the Schema Primer cannot be valid, because in any instance
document validated by this schema, the element containing the <keyref> must
occur as a descendant of the element containing the identity constraint to
which the <keyref> occurs.

<element name="purchaseReport">
     <complexType>
          <sequence>
               <element name="regions" type="r:RegionsType">
                    <keyref name="dummy2" refer="r:pNumKey">
                         <selector xpath="r:zip/r:part"/>
                         <field xpath="@number"/>
                    </keyref>
               </element>

               <element name="parts" type="r:PartsType"/>
          </sequence>
          <attribute name="period"       type="duration"/>
          <attribute name="periodEnding" type="date"/>
     </complexType>

     <unique name="dummy1">
          <selector xpath="r:regions/r:zip"/>
          <field xpath="@code"/>
     </unique>

     <key name="pNumKey">
          <selector xpath="r:parts/r:part"/>
          <field xpath="@number"/>
     </key>
</element>

Given the important role that the Primer plays in helping people to learn
and develop XML Schemas, if this is in fact an erratum I hope that it will
be possible to take speedy measures to make this clear to readers.

Regards,
Neil Graham
XML Parser Development
IBM Toronto Lab
Phone:  416-448-3519, T/L 778-3519
E-mail:  neilg@ca.ibm.com
Received on Friday, 22 June 2001 17:43:59 GMT

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