Make xml element unique using xsd

Question

I have an xml file with the associated xsd below:

The element ContentTitle needs to be unique. I've done some investigating and know to use the xs:unique attribute, but I've yet to get this working correctly with my scenario.

Can anyone offer advice?

  <xs:element name="AllMeta">
    <xs:complexType>
      <xs:sequence>
        <xs:element maxOccurs="unbounded" name="WikiMeta">
          <xs:complexType>
            <xs:sequence>
              <xs:element name="ContentTitle" type="xs:string" minOccurs="1" maxOccurs="1" />
              <xs:element name="PageTitle" type="xs:string" minOccurs="1" maxOccurs="1" />
              <xs:element name="PageMetaDescription" type="xs:string" minOccurs="1" maxOccurs="1" />
            </xs:sequence>
          </xs:complexType>
        </xs:element>
      </xs:sequence>
    </xs:complexType>
  </xs:element>
</xs:schema>

Answer 1

Add a xs:unique element after the xs:complexType for AllMeta :

<?xml version="1.0" encoding="utf-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <xs:element name="AllMeta">
    <xs:complexType>
      <xs:sequence>
        <xs:element maxOccurs="unbounded" name="WikiMeta">
          <xs:complexType>
            <xs:sequence>
              <xs:element name="ContentTitle" type="xs:string" minOccurs="1" maxOccurs="1" />
              <xs:element name="PageTitle" type="xs:string" minOccurs="1" maxOccurs="1" />
              <xs:element name="PageMetaDescription" type="xs:string" minOccurs="1" maxOccurs="1" />
            </xs:sequence>
          </xs:complexType>
        </xs:element>
      </xs:sequence>
    </xs:complexType>
    <xs:unique name="uniqueContentTitle">
      <xs:selector xpath="WikiMeta"/>
      <xs:field xpath="ContentTitle"/>
    </xs:unique>    
  </xs:element>
</xs:schema>

Then, this XML document instance will be valid:

<?xml version="1.0" encoding="UTF-8"?>
<AllMeta xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:noNamespaceSchemaLocation="try.xsd">
  <WikiMeta>
    <ContentTitle>One</ContentTitle>
    <PageTitle></PageTitle>
    <PageMetaDescription></PageMetaDescription>
  </WikiMeta>
  <WikiMeta>
    <ContentTitle>Two</ContentTitle>
    <PageTitle></PageTitle>
    <PageMetaDescription></PageMetaDescription>
  </WikiMeta>
</AllMeta>

And this XML document instance will be invalid:

<?xml version="1.0" encoding="UTF-8"?>
<AllMeta xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:noNamespaceSchemaLocation="try.xsd">
  <WikiMeta>
    <ContentTitle>One</ContentTitle>
    <PageTitle></PageTitle>
    <PageMetaDescription></PageMetaDescription>
  </WikiMeta>
  <WikiMeta>
    <ContentTitle>One</ContentTitle>
    <PageTitle></PageTitle>
    <PageMetaDescription></PageMetaDescription>
  </WikiMeta>
</AllMeta>

And an error message such as the following will be given by the validator (Xerces-J, in this case):

[Error] try.xml:10:37: cvc-identity-constraint.4.1: Duplicate unique value [One] declared for identity constraint "uniqueContentTitle" of element "AllMeta".
c:/gd/usr/kjh/proj/try/xsd/try.xml: 219 ms (9 elems, 1 attrs, 0 spaces, 49 chars)