XML schema for elements with same name but different sub-structure depending on context

Question

I try to define a schema for XML documents I receive.

The documents look like:

<root>
    <items>
        <group name="G-1">
            <item name="I-1"/>
            <item name="I-2"/>
            <item name="I-3"/>
            <item name="I-4"/>
        </group>
    </items>
    <data>
        <group name="G-1" place="here">
            <customer name="C-1">
                <item name="I-1" count="3"/>
                <item name="I-2" count="4"/>
            </customer>
            <customer name="C-2">
                <item name="I-3" count="7"/>
            </customer>
        </group>
    </data>
</root>

I tried XmlSpy and xsd.exe from .NET 2.0. Both created schema definitions which allow below <group> any number of <item> and <customer> elements. But what I'm looking for should restrict <group> below <items> to <item> elements, and <group> below <data> to <customer> elements.

Is this something xml schema is not capable at all?

Answer 1

The key points (see XML Schema Runtime Polymorphism via xsi:type and Abstract Types for complete and correct context/placement/usage) are:

1. Create a base type with (abstract="true" to prevent it from being used directly)

   Note: the ref attribute replaces the name attribute for elements defined elsewhere

   <xs:complexType name="CustomerType" abstract="true" > 
     <xs:sequence> 
       <xs:element ref="cust:FirstName" /> 
       <xs:element ref="cust:LastName" /> 
       <xs:element ref="cust:PhoneNumber" minOccurs="0"/> 
     </xs:sequence> 
     <xs:attribute name="customerID" type="xs:integer" /> 
   </xs:complexType>
   

2. Create two or more derived types by extending or restricting the base type

   <xs:complexType name="MandatoryPhoneCustomerType" > 
     <xs:complexContent> 
       <xs:restriction base="cust:CustomerType"> 
         <xs:sequence> 
           <xs:element ref="cust:FirstName" /> 
           <xs:element ref="cust:LastName" /> 
           <xs:element ref="cust:PhoneNumber" minOccurs="1" /> 
         </xs:sequence> 
       </xs:restriction> 
     </xs:complexContent> 
   </xs:complexType>
   

   and

   <xs:complexType name="AddressableCustomerType" > 
     <xs:complexContent> 
       <xs:extension base="cust:CustomerType"> 
         <xs:sequence> 
           <xs:element ref="cust:Address" /> 
           <xs:element ref="cust:City" /> 
           <xs:element ref="cust:State" /> 
           <xs:element ref="cust:Zip" /> 
         </xs:sequence> 
       </xs:extension> 
     </xs:complexContent> 
   </xs:complexType>
   

3. Reference the base type in an element

   <xs:element name="Customer" type="cust:CustomerType" />
   

4. In your instance XML document, specify the specific derived type as an xsi:type attribute

   <cust:Customer customerID="12345" xsi:type="cust:MandatoryPhoneCustomerType" > 
     <cust:FirstName>Dare</cust:FirstName> 
     <cust:LastName>Obasanjo</cust:LastName> 
     <cust:PhoneNumber>425-555-1234</cust:PhoneNumber> 
   </cust:Customer>
   

   or:

   <cust:Customer customerID="67890" xsi:type="cust:AddressableCustomerType" > 
     <cust:FirstName>John</cust:FirstName> 
     <cust:LastName>Smith</cust:LastName> 
     <cust:Address>2001</cust:Address> 
     <cust:City>Redmond</cust:City> 
     <cust:State>WA</cust:State> 
     <cust:Zip>98052</cust:Zip> 
   </cust:Customer>
   

Answer 2

Yes, XSD can handle this. I generated this schema from Visual Studio 2008 (much faster than doing it by hand) and it will do what you're looking for:

<?xml version="1.0" encoding="utf-8"?>
<xs:schema attributeFormDefault="unqualified" elementFormDefault="qualified" xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <xs:element name="root">
    <xs:complexType>
      <xs:sequence>
        <xs:element name="items">
          <xs:complexType>
            <xs:sequence>
              <xs:element name="group">
                <xs:complexType>
                  <xs:sequence>
                    <xs:element maxOccurs="unbounded" name="item">
                      <xs:complexType>
                        <xs:attribute name="name" type="xs:string" use="required" />
                      </xs:complexType>
                    </xs:element>
                  </xs:sequence>
                  <xs:attribute name="name" type="xs:string" use="required" />
                </xs:complexType>
              </xs:element>
            </xs:sequence>
          </xs:complexType>
        </xs:element>
        <xs:element name="data">
          <xs:complexType>
            <xs:sequence>
              <xs:element name="group">
                <xs:complexType>
                  <xs:sequence>
                    <xs:element maxOccurs="unbounded" name="customer">
                      <xs:complexType>
                        <xs:sequence>
                          <xs:element maxOccurs="unbounded" name="item">
                            <xs:complexType>
                              <xs:attribute name="name" type="xs:string" use="required" />
                              <xs:attribute name="count" type="xs:unsignedByte" use="required" />
                            </xs:complexType>
                          </xs:element>
                        </xs:sequence>
                        <xs:attribute name="name" type="xs:string" use="optional" />
                      </xs:complexType>
                    </xs:element>
                  </xs:sequence>
                  <xs:attribute name="name" type="xs:string" use="required" />
                  <xs:attribute name="place" type="xs:string" use="required" />
                </xs:complexType>
              </xs:element>
            </xs:sequence>
          </xs:complexType>
        </xs:element>
      </xs:sequence>
    </xs:complexType>
  </xs:element>
</xs:schema>