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Taking On The Fleet - XML

This article introduces you to the concept of uniqueness in the XML Schema world, showing you how to use built-in schema constructs to enforce uniqueness within your XML document instances.

TABLE OF CONTENTS:
  1. Doing More With XML Schemas (part 3)
  2. A Day At The Supermarket
  3. Of Fruits And Vegetables
  4. Taking On The Fleet
  5. Breaking The Mold
  6. Two For One
By: Harish Kamath, (c) Melonfire
Rating: starstarstarstarstar / 4
January 30, 2003

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Let's look at another example to better understand how keys and references work. This time, I'll leave the all-too-human world of supermarkets and travel back to that galaxy far, far away, to see exactly what's sitting in the cargo hold of two of the better-known starships in the Star Wars fleet.

<?xml version="1.0" encoding="UTF-8"?> <fleet xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"> <ship name="Jabitha"> <droid type="R2D2">2</droid> <droid type="C3PO">4</droid> </ship> <ship name="Millennium Falcon"> <droid type="R2D2">1</droid> <droid type="C3PO">1</droid> </ship> </fleet>
Here's the schema against which this document would be validated.

<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema"> <xsd:element name="fleet"> <xsd:complexType> <xsd:sequence> <xsd:element name="ship" type="shipType" maxOccurs="unbounded"/> </xsd:sequence> </xsd:complexType> </xsd:element> <xsd:complexType name="droidType"> <xsd:simpleContent> <xsd:extension base="xsd:string"> <xsd:attribute name="type" type="xsd:string"/> </xsd:extension> </xsd:simpleContent> </xsd:complexType> <xsd:complexType name="shipType"> <xsd:sequence> <xsd:element name="droid" type="droidType" maxOccurs="unbounded"/> </xsd:sequence> <xsd:attribute name="name" type="xsd:string"/> </xsd:complexType> </xsd:schema>
Now, let's suppose I wanted to add a couple more droids to the Falcon. Sure, I could add another <ship> element with the same name...or I could do the smart thing, and add another <droid> element to the existing definition. As discussed in the previous example, the latter option is much cleaner, and also fairly easy to implement via the <xsd:unique> element. Here's the relevant snippet of the updated schema definition:

<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema"> <!-- snip --> <xsd:element name="fleet"> <xsd:complexType> <xsd:sequence> <xsd:element name="ship" type="shipType" maxOccurs="unbounded"/> </xsd:sequence> </xsd:complexType> <xsd:unique name="NoRedefines"> <xsd:selector xpath="ship"/> <xsd:field xpath="@name"/> </xsd:unique> </xsd:element> <!-- snip --> </xsd:schema>
In order to verify this, you can try creating two <ship> elements with the same "name", and seeing your XML validator throw up all over the screen. It's always fun to watch, and it doesn't hurt anything!

 
 
>>> More XML Articles          >>> More By Harish Kamath, (c) Melonfire
 

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