What Is "key" Identity-Constraint?

XSD Tutorials - Herong's Tutorial Examples

∟Identity-Constraints: unique, key and keyref

∟What Is "key" Identity-Constraint?

This section describes the 'key' identity-constraint, where a combination of certain sub elements or attributes is defined as a key identity, which must have values and values must be unique.

What Is "key" Identity-Constraint? A "key" Identity-Constraint is a constraint on an XML element, where a combination of certain sub elements or attributes is defined as a key identity, which must have values and values must be unique.

"key" identity-constraint in XML document is very similar to the concept of PRIMARY KEY constraint on multiple columns in a database table.

Here is the syntax on how a "key" identity-constraint should be defined in XSD schema:

 <xs:element name="parent" ...>
  ...
  <xs:key name="constraint-name">
   <xs:selector xpath="sub-element-path"/>
   <xs:field xpath="field-1-path"/>
   <xs:field xpath="field-2-path"/>
   ...
  </xs:key>
 </xs:element>

Note that:

The "key" declaration must be placed inside the "element" statement which declares the parent element where the constraint will be applied.
"key[@name]" is required to provide a name for this key identity constraint.
"selector" is required to specify a relative XPath to match a sequence of sub elements from which identity values will be constructed and validated.
At least 1 "field" is required.
Each "field" specifies a relative XPath pointing to an attribute or a simple element content, which will be used as part of the identity construction.

For example, I have the following XML document, key_identity.xml:

<?xml version="1.0" encoding="utf-8"?>
<!-- key_identity.xml
 - Copyright (c) 2002-2013 HerongYang.com. All Rights Reserved.
-->
<questions>
 <question>What is Java?</question>
 <question code="John">Who is John?</question>
 <question code="Jim">Where is Jim?</question>
 <question code="Jim">What is Jim doing?</question>
</questions>

For this XML document, I want to have an XSD schema that has an identity-constraint to ensure values in "code" attributes can be used as a key identity. Here is my XSD schema, key_identity.xsd:

<?xml version="1.0" encoding="utf-8"?>
<!-- key_identity.xsd
 - Copyright (c) 2002-2013 HerongYang.com. All Rights Reserved.
-->
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
 <xs:element name="questions" type="questionsType">
  <xs:key name="MyKey">
   <xs:selector xpath="./question"/>
   <xs:field xpath="@code"/>
  </xs:key>
 </xs:element>

 <xs:complexType name="questionsType">
  <xs:sequence>
   <xs:element name="question" maxOccurs="unbounded">
    <xs:complexType>
     <xs:simpleContent>
      <xs:extension base="xs:string">
       <xs:attribute name="code" type="xs:string"/>
      </xs:extension>
     </xs:simpleContent>
    </xs:complexType>
   </xs:element>
  </xs:sequence>
 </xs:complexType>
</xs:schema>

The identify-constraint "MyKey" is specified inside the "questions" element definition statement. Let's use the "jaxp.TypeInfoWriter" sample program from Xerces2 to validate the XML document to see if the constraint works or not:

herong> java_xerces jaxp.TypeInfoWriter -a key_identity.xsd
^^^ -i key_identity.xml

setDocumentLocator(systemId="file:///C:/herong/key_identity.xml", ...
startDocument()
 startElement(name="questions",type="questionsType",attributes={})
  startElement(name="question",type="#AnonType_questionquestionsTy...
[Error] key_identity.xml:6:37: cvc-identity-constraint.4.2.1.a: Element
^^^ "questions" has no value for the key "MyKey".
  endElement(name="question")
  startElement(name="question",type="#AnonType_questionquestionsTy...
  endElement(name="question")
  startElement(name="question",type="#AnonType_questionquestionsTy...
  endElement(name="question")
[Error] key_identity.xml:9:23: cvc-identity-constraint.4.1: Duplicate
^^^ unique value [Jim] declared for identity constraint
^^^ "MyKey" of element "questions".
  startElement(name="question",type="#AnonType_questionquestionsTy...
  endElement(name="question")
 endElement(name="questions")
endDocument()

I think the "MyKey" constraint worked correctly, and here is what happened:

When the parser reached the "questions" element, it started to apply the "MyKey" constraint.
The first sub element <question>What is Java?</question> matched the "selector" condition, xpath="./question". The parser tried to get the "field" value using xpath="@code". In the case, it was a missing value. Validation failed.
The second sub element <question code="John">Who is John?</question> matched the "selector" condition, xpath="./question". The parser tried to get the "field" value using xpath="@code". In the case, it was a unique value, "John". Validation passed.
The third sub element <question code="Jim">Where is Jim?</question> matched the "selector" condition, xpath="./question". The parser tried to get the "field" value using xpath="@code". In the case, it was a duplicate value, "Jim", because the next sub element also has the same attribute code="Jim". Validation failed.