XSD Tutorial (3): Mis

In last blog, we have some problems and their solution, in this thread, we are going to solve more specific problems.

Define `id` Like Attribute?

<xs:attribute name="id" type="xs:ID"/>

Define `ref` Like Attribute?

<xs:attribute name="ref" type="xs:IDREF"/>

Default Value for Element?

Short answer is no, xsd doesn’t provide this functionality.

Attributes can be set with a default value because it is relative simple, while elements can nested and combined, which is much more complex and hard to provide a default content.

As far as we know, there also exists no alternatives to provide a default value for a missing element. In order to provide this functionality, we add much more code when combined with jaxb.

Define `key` & `keyref`

We define them in following steps:

First, we must define key & keyref in element body;
Second, definekey in following format:

<key
  id = ID
  name = NCName
  {any attributes with non-schema namespace . . .}>
  Content: (annotation?, (selector, field+))
</key>


<xs:key name="key">
    <xs:selector xpath=".//state"/>
    <xs:field xpath="code"/>
</xs:key>

Then, simple XPath syntax rule:

Expression	Description
nodename	Selects all nodes with the name “nodename”
/	Selects from the root node
//	Selects nodes in the document from the current node that match the selection no matter where they are
.	Selects the current node
..	Selects the parent of the current node
@	Selects attributes

A simple key & keyref example (in which vehicle reference it’s father node):

<xs:element name="vehicle">
    <xs:complexType>
        <xs:attribute name="state" type="xs:integer"/>
    </xs:complexType>
</xs:element>

<xs:element name="state">
    <xs:complexType>
        <xs:sequence>
            <xs:element name="code" type="xs:integer"/>
            <xs:element ref="vehicle"/>
        </xs:sequence>
    </xs:complexType>
</xs:element>

<xs:element name="root">
    <xs:complexType>
        <xs:sequence>
            <xs:element ref="state"/>
        </xs:sequence>
    </xs:complexType>

    <xs:key name="state">
        <!-- states are keyed by their code -->
        <xs:selector xpath=".//state"/>
        <xs:field xpath="code"/>
    </xs:key>

    <xs:keyref name="vehicleState" refer="state">
        <!-- every vehicle refers to its state -->
        <xs:selector xpath=".//vehicle"/>
        <xs:field xpath="@state"/>
    </xs:keyref>
</xs:element>

<state>
    <code>12</code>
    <vehicle state="12"/>
</state>

A more complicated xsd/xml code can be found in my git repo.

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