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7.3. Simple Content Models

We will start by looking at complex types containing simple content because they are closest to simple types, which we've seen recently, and they also provide an easier transition to the more complex world of complex contents. We will not discuss the creation and derivation of simple types, already covered in Chapter 5, "Creating Simple Datatypes", but instead will focus on complex types' simple content models (i.e., elements having only text nodes and attributes) and study how they are created and derived.

7.3.2. Derivation from Simple Contents

Complex types provide a number of options for extending simple content models.

7.3.2.2. Derivation by restriction

The derivation by restriction of simple content complex types is a feature at the border between the two parts of W3C XML Schema (Part 1: Structure and Part 2: Datatypes). It's also very similar to the derivation by restriction of simple datatypes, discussed in Chapter 6, "Using Regular Expressions to Specify Simple Datatypes". The only difference between the derivations by restriction in these two contexts is that the derivation by restriction of a simple content complex type allows not only restriction of the scope of the text node, but also the restriction of the scope of the attribute. This restriction follows the same principle as the restriction of a simple type: any instance structure deemed valid per the restricted type must also be valid per the base type (with the exception already mentioned for the xs:whiteSpace facet).

The syntax used to restrict the text child is the same as the syntax used to derive simple types by restriction. The facets are the same as well. These facets must be followed by the new list of attributes, which may have different types as long as they are derived from the types of the attributes from the base type. Attributes that are not mandatory in the base type can be specified in the new list as "prohibited," and attributes that are not included are considered unchanged. Following are some examples of derivations that start from a simple content datatype equivalent to the content model just shown:

<xs:complexType name="tokenWithLangAndNote">
  <xs:simpleContent>
    <xs:extension base="xs:token">
      <xs:attribute name="lang" type="xs:language"/>
      <xs:attribute name="note" type="xs:token"/>
    </xs:extension>
  </xs:simpleContent>
</xs:complexType>

We can first show how to restrict the length of the text node, as we've done for simple types:

<xs:element name="title">
  <xs:complexType>
    <xs:simpleContent>
      <xs:restriction base="tokenWithLangAndNote">
        <xs:maxLength value="255"/> 
         <xs:attribute name="lang" type="xs:language"/>
        <xs:attributename="note" type="xs:token"/>
      </xs:restriction>
    </xs:simpleContent>
  </xs:complexType>
</xs:element>

To remove the note attribute from the element title, we declare note to be prohibited in the list of attributes in the restriction:

<xs:element name="title">
  <xs:complexType>
    <xs:simpleContent>
      <xs:restriction base="tokenWithLangAndNote">
        <xs:maxLength value="255"/>
        <xs:attribute name="lang" type="xs:language"/>
        <xs:attribute name="note" use="prohibited"/>
      </xs:restriction>
    </xs:simpleContent>
  </xs:complexType>
</xs:element>

We can also restrict the datatype by restricting its attributes. For instance, if we want to restrict the number of possible languages, we can do it directly in the definition of the lang attribute in the derived type:

<xs:element name="title">
  <xs:complexType>
    <xs:simpleContent>
      <xs:restriction base="tokenWithLangAndNote">
        <xs:maxLength value="255"/>
        <xs:attribute name="lang">
          <xs:simpleType>
            <xs:restriction base="xs:language">
              <xs:enumeration value="en"/>
              <xs:enumeration value="es"/>
            </xs:restriction>
          </xs:simpleType>
        </xs:attribute>
      </xs:restriction>
    </xs:simpleContent>
  </xs:complexType>
</xs:element>


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