Question

I am trying to achieve an XML structure equivalent to this:

<worker>
<name>Adam</name>
<project>
<name>Foo</name>
</project>
<project>
<name>Bar</name>
</project>
</worker>
<worker>
<name>Eve</name>
<project>
<name>Foo</name>
</project>
</worker>
<worker>
<name>Lizzie</name>
<project>
<name>Bar</name>
</project>
</worker>

but without duplicating the tag contents. How can I introduce links between projects and workers in a way which would be validated by the parser and what would the required .xsd file look like?

Answer 1

XSD has support for identity relationships through the keyref and key elements. I was going to come up with an example but found this one covers it well. I’ve copied the sample below. Notice that each item has a @number that corresponds to a product in the products section of the document. This is enforced in the xsd through the OrderType definition.

<order xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xsi:noNamespaceSchemaLocation="chapter17.xsd">
  <number>123ABBCC123</number>
  <items>
    <shirt number="557">
      <quantity>1</quantity>
      <color value="blue"/>
    </shirt>
    <shirt number="557">
      <quantity>1</quantity>
      <color value="sage"/>
    </shirt>
    <hat number="563">
      <quantity>1</quantity>
    </hat>
  </items>
  <products>
    <product>
      <number>557</number>
      <name>Short-Sleeved Linen Blouse</name>
      <price currency="USD">29.99</price>
    </product>
    <product>
      <number>563</number>
      <name>Ten-Gallon Hat</name>
      <price currency="USD">69.99</price>
    </product>
  </products>
</order>


<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">

  <xs:element name="order" type="OrderType">
    <xs:keyref name="prodNumKeyRef" refer="prodNumKey">
      <xs:selector xpath="items/*"/>
      <xs:field xpath="@number"/>
    </xs:keyref>
    <xs:key name="prodNumKey">
      <xs:selector xpath=".//product"/>
      <xs:field xpath="number"/>
    </xs:key>
  </xs:element>

  <xs:complexType name="OrderType">
    <xs:sequence>
      <xs:element name="number" type="xs:string"/>
      <xs:element name="items" type="ItemsType"/>
      <xs:element name="products" type="ProductsType"/>
    </xs:sequence>
  </xs:complexType>

  <xs:complexType name="ItemsType">
    <xs:choice maxOccurs="unbounded">
      <xs:element name="shirt" type="ProductOrderType"/>
      <xs:element name="hat" type="ProductOrderType"/>
    </xs:choice>
  </xs:complexType>

  <xs:complexType name="ProductOrderType">
    <xs:sequence>
      <xs:element name="quantity" type="xs:integer"/>
      <xs:element name="color" type="ColorType" minOccurs="0"/>
    </xs:sequence>
    <xs:attribute name="number" type="xs:integer"/>
  </xs:complexType>

  <xs:complexType name="ProductsType">
    <xs:sequence>
      <xs:element name="product" type="ProductType"
                   maxOccurs="unbounded"/>
    </xs:sequence>
  </xs:complexType>

  <xs:complexType name="ProductType">
    <xs:sequence>
      <xs:element name="number" type="xs:integer"/>
      <xs:element name="name" type="xs:string"/>
      <xs:element name="price" type="PriceType"/>
    </xs:sequence>
  </xs:complexType>

  <xs:complexType name="ColorType">
    <xs:attribute name="value" type="xs:string"/>
  </xs:complexType>

  <xs:complexType name="PriceType">
    <xs:simpleContent>
      <xs:extension base="xs:decimal">
        <xs:attribute name="currency" type="xs:token"/>
      </xs:extension>
    </xs:simpleContent>
  </xs:complexType>

</xs:schema>