Question

Taking the W3C vehicle XSD as an example:

<schema xmlns="http://www.w3.org/2001/XMLSchema"
           targetNamespace="http://cars.example.com/schema"
           xmlns:target="http://cars.example.com/schema">

  <complexType name="Vehicle" abstract="true"/>

  <complexType name="Car">
    <complexContent>
      <extension base="target:Vehicle"/>
      ...
    </complexContent>
  </complexType>

  <complexType name="Plane">
    <complexContent>
      <extension base="target:Vehicle"/>
      <sequence>
        <element name="wingspan" type="integer"/>
      </sequence>
    </complexContent>
  </complexType>      
</schema>

, and the following definition of ‘meansOfTravel’:

<complexType name="MeansOfTravel">
  <complexContent>
    <sequence>        
      <element name="transport" type="target:Vehicle"/>        
    </sequence>
  </complexContent>
</complexType>

<element name="meansOfTravel" type="target:MeansOfTravel"/>

With this definition you need to specify the type of your instance using xsi:type, like this:

<meansOfTravel>
  <transport xsi:type="Plane">
     <wingspan>3</wingspan>
  </transport>
</meansOfTravel>

I would just like to acheive a ‘name of type’ – ‘name of element’ mapping so that this could be replaced with just

<meansOfTravel>
  <plane>
    <wingspan>3</wingspan>
  </plane>
</meansOfTravel>

The only way I could do this until now is by making it explicit:

<complexType name="MeansOfTravel">
  <sequence>        
    <choice>
      <element name="plane" type="target:Plane"/>
      <element name="car" type="target:Car"/>         
    </choice>
  </sequence>
</complexType>

<element name="meansOfTravel" type="target:MeansOfTravel"/>

But this means that I have to list all possible sub-types in the ‘MeansOfTravel’ complex type. Is there no way of making the XML parser assume that you mean a ‘Plane’ if you call the element ‘plane’? Or do I have to make the choice explicit? I would just like to keep my design DRY – if you have any other suggestions (like groups or so) – i am all ears.

Answer 1

There is a common design pattern around this, you can use sub-types (as you are already doing), and elements in a substitution group. Elements in the substitution group have to be of a sub-type of the element they are substituted for.

Unfortuntaly, substitution group elements need to be defined as global elements. So you would have this:

<complexType name="MeansOfTravel">
  <complexContent>
    <sequence>        
      <element ref="transport"/>
    </sequence>
  </complexContent>
</complexType>

<element name="transport" type="target:Vehicle"/>
<element name="plane" type="target:Plane" substitutionGroup="target:transport"/>

Then, in your XML document you can use:

<meansOfTravel>
    <plane>...</plane>
</meansOfTravel>

More info on substitution groups here. And no, unfortunately the parser cannot guess this, so you still have to list the elements 🙁 There is one advantage over a choice though: the schema can be extended externally, by importing it, without modifying it. The choice could not be extended.