Extending xsd schema with "choice" model group

[Jaikiran Pai]

I am playing with some xsd stuff these days. One of the things i am trying to do is extend an existing xsd to provide more elements in the new one. The base xsd looks like this:

<?xml version="1.0" encoding="UTF-8"?>
<xsd:schema xmlns="http://www.w3.org/2001/XMLSchema"
            targetNamespace="http://www.myapp.org/xml/ns/myapp"
            xmlns:myapp="http://www.myapp.org/xml/ns/myapp"
            xmlns:xsd="http://www.w3.org/2001/XMLSchema"
            elementFormDefault="qualified"
            attributeFormDefault="unqualified"
            version="1.0">

	<xsd:element name="graphics" type="graphicsType/>

	<xsd:complexType name="graphicsType">
		<xsd:sequence>
			<xsd:element name="colours" type="coloursType"/>
		</xsd:sequence>
	</xsd:complexType>
	

	<xsd:complexType name="coloursType">
	    <xsd:choice maxOccurs="unbounded">
      		<xsd:element name="red">
			<xsd:complexType />
		</xsd:elemen>
		<xsd:element name="blue">
			<xsd:complexType />
		</xsd:element>          
	   </xsd:choice>
	</xsd:complexType>		
</xsd:schema>

The extended xsd is as follows:

<?xml version="1.0" encoding="UTF-8"?>
<xsd:schema xmlns="http://www.w3.org/2001/XMLSchema"
            targetNamespace="http://www.myapp.org/xml/ns/myapp"
            xmlns:myapp="http://www.myapp.org/xml/ns/myapp"
            xmlns:xsd="http://www.w3.org/2001/XMLSchema"
            elementFormDefault="qualified"
            attributeFormDefault="unqualified"
            version="1.0">

	<xsd:element name="extended-graphics" type="extended-graphicsType/>
	
	<xsd:complexType name="extended-graphicsType">
		<xsd:extension base="graphicsType">
			<xsd:sequence>
				<xsd:element name="brush" type="xsd:string"/>
			</xsd:sequence>
		</xsd:extension>
	</xsd:complexType>

	<xsd:complexType name="more-coloursType" >
		<xsd:extension base="coloursType">
	    		<xsd:choice maxOccurs="unbounded">
      				<xsd:element name="dark-red">
					<xsd:complexType />
				</xsd:elemen>
				<xsd:element name="dark-blue">
					<xsd:complexType />
				</xsd:element>        
	   		</xsd:choice>
		</xsd:extension>
	</xsd:complexType>
		
</xsd:schema>

So here i have a "coloursType" with a choice of "red" and "blue" and the extended "more-coloursType" has the additional "dark-red" and "dark-blue". Then in my xml i am trying to do this:

<extended-graphics xmlns="http://www.myapp.org/xml/ns/myapp"
             xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
             xmlns:myapp="http://www.myapp.org/xml/ns/myapp">

	<colours xsi:type="more-coloursType">
		<dark-red/>
	</colour>                 
                 
</extended-graphics>     

So in the xml, i am trying to use the extended "more-coloursType" and specifying a "dark-red" value. But this fails validation, with a message saying that "dark-red" is not allowed in this place. It starts working if i first have a normal colour (either red or blue) and then have dark-red:

<colours xsi:type="more-coloursType">
		<blue/>
		<dark-red/>
	</colour>   

My understanding was with this kind of extension, with the "choice" elements i would have something like:

(red | blue)* (dark-red | dark-blue)*

but this doesn't seem to be the case. Am i missing something? Any changes that i'll have to make to my xsd to allow:

     <colours xsi:type="more-coloursType">
		<dark-red/>
	</colour>