Example instance document representing the DessertMenu in VS .NET

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Listing 2-2-5 Example instance document representing the DessertMenu
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<DessertMenu xmlns:xsi="http://wwww3org/2001/XMLSchema-instance" xmlns="http://schemasdevxpertcom/examples" xmlns:dx="http://schemasdevxpertcom/examples"> <Item1 xsi:type="dx:IceCream"> <Calories>3000</Calories> <Flavor>Oreo cookie</Flavor> </Item1> <Item2 xsi:type="dx:IceCream"> <Calories>3000</Calories> <Flavor>Oreo cookie</Flavor> </Item2>
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<Item3 xsi:type="dx:Cake"> <Calories>4000</Calories> <IsChocolate>true</IsChocolate> </Item3> </DessertMenu>
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XSD also supports the concept of abstract base types In the above example, if Dessert was a MustInherit (abstract) class, the corresponding XSD type would be the same as shown before with the abstract attribute set to true:
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<!-- the Dessert base type made abstract --> <complexType name="Dessert" abstract="true"> <sequence> <element name="Calories" type="long" /> </sequence> </complexType>
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Just like you cannot instantiate an object from a MustInherit class, you also cannot have an element of an abstract type in an XML document Therefore by making Dessert abstract, you require that all <Item> elements in the DessertMenu be of IceCream or Cake type, ie it is now illegal for an <Item> element to not have the xsi:type attribute
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XSD lets you explicitly indicate that an element is nil (Nothing in VB) as opposed to empty (ie no content) For example, if you were out of chocolate cake and the DessertMenu object had only two items, you d expect Item3 to be Nothing To express this in the schema and instance document, you perform two steps: First, modify the schema so that the item elements are nillable:
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<!-- the DessertMenu --> <element name="DessertMenu"> <complexType> <sequence> <element name="Item1" type="dx:Dessert" nillable="true" /> <element name="Item2" type="dx:Dessert" nillable="true"/> <element name="Item3" type="dx:Dessert" nillable="true"/>
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</sequence> </complexType> </element>
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Then you modify the XML instance document by adding xsi:nil="true" to <Item3> thereby indicating that it is nil:
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<Item3 xsi:nil="true"></Item3>
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Notice that I removed the xsi:type attribute from <Item3> This is because if an object reference is Nothing, do you know or care what type it really is See the chapter s opening quote for a humorous analogy on this topic
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Uniqueness and keys
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When you use XML to represent your application s data, you ll want the ability to specify uniqueness constraints similar to the concept of a primary key common in database technologies XSD provides the <unique> element which you can use to define uniqueness constraints on element or attribute values The <unique> element contains a <selector> element which lets you specify a restricted XPath expression to indicate the scope of the uniqueness constraint In the relational database analogy, this is similar to indicating the table on which you are defining the primary key The <unique> element also contains one or more <field> elements that indicate which fields form the primary key Just like in a relational database, the primary key can be made up of multiple fields, XSD also lets you specify more than one <field> element within a <unique> constraint Listing 2-6 shows an example schema fragment that defines a uniqueness constraint on the invoiceID attribute of <invoice> elements Listing 2-2-6 Defining a uniqueness constraint for the invoiceID
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<xsd:element name="invoices"> <xsd:complexType> <xsd:sequence> <xsd:element name="invoice" maxOccurs="unbounded"> <xsd:complexType> <xsd:sequence> <!-- invoiceID must be unique --> <xsd:element name="invoiceID" type="xsd:int" /> <xsd:element name="invoiceNumber" type="xsd:string" />
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<xsd:element name="supplierID" type="xsd:int" /> <xsd:element name="invoiceDate" type="xsd:dateTime" /> <xsd:element name="amtDue" type="xsd:decimal" /> </xsd:sequence> </xsd:complexType> </xsd:element> <xsd:sequence> </xsd:complexType> <!-- a uniqueness constraint --> <xsd:unique name="PKInvoice"> <!-- applies to invoice elements --> <xsd:selector xpath="//invoice" /> <!-- invoiceID is the element whose value must be unique --> <xsd:field xpath="invoiceID" /> </xsd:unique> </xsd:element>
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Note that the uniqueness constraint for <invoice> is defined within the <invoices> element declaration which is the container for <invoice> elements You can also define key-based relations similar to the foreign key-primary key relations found in relational databases Defining relations is a two step process: First define the key using a <key> element then reference that key using a <keyref> element Listing 2-7 shows an example of defining a relation between the part number in an invoice item and the part number of a product Listing 2-2-7 Defining relations with key and keyref
<xsd:element name="products"> <xsd:complexType> <xsd:sequence> <!-- the product element --> <xsd:element name="product" maxoccurs="unbounded">