Table of contents

Appendices

2.7 Building Content Models

The definitions of complex types in the purchase order schema all declare sequences of elements that must appear in the instance document. The occurrence of individual elements declared in the so-called content models of these types may be optional, as indicated by a 0 value for the attribute minOccurs (e.g. in comment), or be otherwise constrained depending upon the values of minOccurs and maxOccurs. XML Schema also provides constraints that apply to groups of elements appearing in a content model. These constraints mirror those available in XML 1.0 plus some additional constraints. Note that the constraints do not apply to attributes.

XML Schema enables groups of elements to be defined and named, so that the elements can be used to build up the content models of complex types (thus mimicking common usage of parameter entities in XML 1.0). Un-named groups of elements can also be defined, and along with elements in named groups, they can be constrained to appear in the same order (sequence) as they are declared. Alternatively, they can be constrained so that only one of the elements may appear in an instance.

ref17To illustrate, we introduce two groups into the PurchaseOrderType definition from the purchase order schema so that purchase orders may contain either separate shipping and billing addresses, or a single address for those cases in which the shippee and billee are co-located:

NOTE: 

Nested Choice and Sequence Groups

<xsd:complexType name="PurchaseOrderType">
  <xsd:sequence>
    <xsd:choice>
      <xsd:group   ref="shipAndBill"/>
      <xsd:element name="singleUSAddress" type="USAddress"/>
    </xsd:choice>
    <xsd:element ref="comment" minOccurs="0"/>
    <xsd:element name="items"  type="Items"/>
  </xsd:sequence>
  <xsd:attribute name="orderDate" type="xsd:date"/>
</xsd:complexType>

<xsd:group id="shipAndBill">
  <xsd:sequence>
    <xsd:element name="shipTo" type="USAddress"/>
    <xsd:element name="billTo" type="USAddress"/>
  </xsd:sequence>
</xsd:group>

The choice group element allows only one of its children to appear in an instance. One child is an inner group element that references the named group shipAndBill consisting of the element sequence shipTo, billTo, and the second child is a singleUSAddress. Hence, in an instance document, the purchaseOrder element must contain either a shipTo element followed by a billTo element or a singleUSAddress element. The choice group is followed by the comment and items element declarations, and both the choice group and the element declarations are children of a sequence group. The effect of these various groups is that the address element(s) must be followed by comment and items elements in that order.

ref18There exists a third option for constraining elements in a group: All the elements in the group may appear once or not at all, and they may appear in any order. The all group (which provides a simplified version of the SGML &-Connector) is limited to the top-level of any content model. Moreover, the group's children must all be individual elements (no groups), and no element in the content model may appear more than once, i.e. the permissible values of minOccurs and maxOccurs are 0 and 1. For example, to allow the child elements of purchaseOrder to appear in any order, we could redefine PurchaseOrderType as:

NOTE: 

An 'All' Group

<xsd:complexType name="PurchaseOrderType">
  <xsd:all>
    <xsd:element name="shipTo" type="USAddress"/>
    <xsd:element name="billTo" type="USAddress"/>
    <xsd:element ref="comment" minOccurs="0"/>
    <xsd:element name="items"  type="Items"/>
  </xsd:all>
  <xsd:attribute name="orderDate" type="xsd:date"/>
</xsd:complexType>

By this definition, a comment element may optionally appear within purchaseOrder, and it may appear before or after any shipTo, billTo and items elements, but it can appear only once. Moreover, the stipulations of an all group do not allow us to declare an element such as comment outside the group as a means of enabling it to appear more than once. XML Schema stipulates that an all group must appear as the sole child at the top of a content model. In other words, the following is illegal:

NOTE: 

Illegal Example with an 'All' Group

<xsd:complexType name="PurchaseOrderType">
  <xsd:sequence>
    <xsd:all>
      <xsd:element name="shipTo" type="USAddress"/>
      <xsd:element name="billTo" type="USAddress"/>
      <xsd:element name="items"  type="Items"/>
    </xsd:all>
    <xsd:sequence>
      <xsd:element ref="comment" minOccurs="0" maxOccurs="unbounded"/>
    </xsd:sequence>
  </xsd:sequence>
  <xsd:attribute name="orderDate" type="xsd:date"/>
</xsd:complexType>

Finally, named and un-named groups that appear in content models (represented by group and choice, sequence, all respectively) may carry minOccurs and maxOccurs attributes. By combining and nesting the various groups provided by XML Schema, and by setting the values of minOccurs and maxOccurs, it is possible to represent any content model expressible with an XML 1.0 DTD. Furthermore, the all group provides additional expressive power.