Re: XML Schema question on inheritance

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

Hi,

James Williams - Sun East Coast IR Development wrote:

> Is it possible to derive a complexType from another complexType
> if they both use the <all> content group ?
>
> Here is an example of what I want to do:
>
>     <complexType name="parentType">
>                 <all>
>                     <element name="x" type="string"/>
>                     <element name="y" type="string"/>
>                 </all>
>     </complexType>
>
>     <complexType name="derivedType">
>         <complexContent>
>             <extension base="parentType">
>                 <all>
>                     <element name="z" type="string"/>
>                 </all>
>             </extension>
>         </complexContent>
>     </complexType>
>
> Xerces 1.4.0  won't allow this, and I'm wondering if this is a bug, or
> something the spec won't allow.

I don't believe this is valid according to the restriction imposed on the
<xsd:all> group. I haven't found the relevant sections in the Structure
spec [1] but the Primer says [2]:

"When a complex type is derived by extension, its effective content model
is the content model of the base type plus the content model specified in
the type derivation. Furthermore, the two content models are treated as
two children of a sequential group."

This means that the above derivation can be written as a single content
model:

<complexType name="parentType">
   <sequence>
      <all>
         <element name="x" type="string"/>
         <element name="y" type="string"/>
      </all>
      <all>
         <element name="z" type="string"/>
      </all>
   </sequence>
</complexType>

According to the constraints imposed on the all group (must be the sole
child at the top of a content model) this is illegal and that would mean
that the all group cannot be used in a type derivation by extension.

Cheers,
/Eddie

[1] http://www.w3.org/TR/xmlschema-1
[2] http://www.w3.org/TR/xmlschema-0/#DerivExt