xsd for an element must have an attribute depend on another attribute value

Question

I have a tag having attributes name required , link='notusing' required and replaced_with (it should be optional only link value is not a string of 'notusing').

Also xsd has to check whether the xml has font with a name that is specified in replaced_with.

Example:

<fonts>
<font name='font1' link='inuse'/>
<font name='font2' link='inuse'/>
<font name='font3' link='notusing' replaced_with="font2"/>
</fonts>

How can i write the xsd for this.

Thanks Samba Damerla

Answer 1

I don't think it's possible to enforce that replaced_with can only occur if link='notusing'. You can make replaced_with optional using minOccurs='0', but that's about it.

If you are able to change the structure of the XML file, you could do something like this instead:

<?xml version="1.0" encoding="utf-8" ?>
<fontData
  xmlns="someNamespace"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="someNamespace XMLSchema1.xsd">
  <fonts>
    <font name="font1" />
    <font name="font2" />
  </fonts>
  <obsoleteFonts>
    <font name="font3" replaced_with="font2" />
  </obsoleteFonts>
</fontData>

And then you could use key and keyref to enforce that the name of any font within obsoleteFonts exists in fonts.

Here's how the XSD file would look to enforce this XML format:

<?xml version="1.0" encoding="utf-8"?>
<xs:schema
  xmlns="someNamespace"
  xmlns:tns="someNamespace"
  xmlns:xs="http://www.w3.org/2001/XMLSchema"
  targetNamespace="someNamespace"
  elementFormDefault="qualified">
  <xs:element name="fontData" type="fontData">
    <xs:key name="fontKey">
      <xs:selector xpath="tns:fonts/tns:font" />
      <xs:field xpath="@name" />
    </xs:key>
    <xs:keyref name="obsoleteFontToFontKeyRef" refer="fontKey">
      <xs:selector xpath="tns:obsoleteFonts/tns:font" />
      <xs:field xpath="@replaced_with" />
    </xs:keyref>
  </xs:element>
  <xs:complexType name="fontData">
    <xs:sequence>
      <xs:element name="fonts" type="fonts" />
      <xs:element name="obsoleteFonts" type="obsoleteFonts" />
    </xs:sequence>
  </xs:complexType>
  <xs:complexType name="fonts">
    <xs:sequence>
      <xs:sequence>
        <xs:element name="font" type="font" maxOccurs="unbounded" />
      </xs:sequence>
    </xs:sequence>
  </xs:complexType>
  <xs:complexType name="obsoleteFonts">
    <xs:sequence>
      <xs:sequence>
        <xs:element name="font" type="obsoleteFont" maxOccurs="unbounded" />
      </xs:sequence>
    </xs:sequence>
  </xs:complexType>
  <xs:complexType name="font">
    <xs:attribute name="name" type="xs:string" />
  </xs:complexType>
  <xs:complexType name="obsoleteFont">
    <xs:attribute name="name" type="xs:string" />
    <xs:attribute name="replaced_with" type="xs:string" />
  </xs:complexType>
</xs:schema>

I tested this with the Visual Studio schema validator, but hopefully it will work properly in whatever technology you are using.

Answer 2

In XML schema (XSD), you cannot express this kind of requirement / restriction. XML schema is all about structure, and you cannot use values of elements or attributes to influence the structure of other elements or attributes. Can't be done, it's just not part of the XML schema standard.

If you need this kind of checks, have a look at Schematron, a different kind of XML validation framework.