XSD: xs:sequence & xs:choice combination for xs:extension elements?

Question

Hi,

My question is about defining an XML schema that will validate the following sample XML:

<rules>
    <other>...</other>
    <bool>...</bool>
    <other>...</other>
    <string>...</string>
    <other>...</other>
</rules>

The order of the child nodes does not matter. The cardinality of the child nodes is 0..unbounded.

All the child elements of the rules node have a common base type, rule, like so:

<xs:complexType name="booleanRule">
    <xs:complexContent>
        <xs:extension base="rule">
            ...
        </xs:extension>
    </xs:complexContent>
</xs:complexType>

<xs:complexType name="stringFilterRule">
    <xs:complexContent>
        <xs:extension base="filterRule">
            ...
        </xs:extension>
    </xs:complexContent>
</xs:complexType>

My current attempt at defining the schema for the rules node is below. However,

1. Can I nest xs:choice within xs:sequence? If, where do I specify the maxOccurs="unbounded" attribute?

2. Is there a better way to do this, such as an xs:sequence which specifies only the base type of its child elements?

       <xs:element name="rules">
           <xs:complexType>
               <xs:sequence>
                   <xs:choice>
                       <xs:element name="bool" type="booleanRule" />
                       <xs:element name="string" type="stringRule" />
                       <xs:element name="other" type="someOtherRule" />
                   </xs:choice>
               </xs:sequence>
           </xs:complexType>
       </xs:element>
   

Answer 1

1. Yes, you can. You can specify maxOccurs="unbounded" where it better suits you. You can put it on xs:choice, for instance (I think that's what you're asking).
2. You can use substitution groups. See this schema for example. ogc:expression is an abstract base element which may be substituted by different concrete expressions (ogc:Add and so on).