An element with more than one possible type

["pau carre" <pau.carre@gmail.com>]

I am trying to code a XML Schema (XSD file) for a web service in such a way:

<items>
   <item name = "water" volume ="22">
   <item name = "meat" weight = "10" >
</items>

It is to say, if name is "water" , then use the "volume" attribute.
When name is "meat" then use "weight" attribute.

What I have done is to define a simple type for name attribute

<xs:simpleType name="nameType">
        <xs:restriction base="xs:string">
                <xs:enumeration value="water"/>
                <xs:enumeration value="meat"/>
        </xs:restriction>
</xs:simpleType>

Then I defined an abstract complex type for "item" element:

<xs:complexType name="itemType" abstract = "true">
        <xs:attribute name = "name" type = "nameType"/>
</xs:complexType>

I restricted the "name" attribute to "meat":

<xs:complexType name="itemTypeMeat">
        <xs:complexContent>
                <xs:restriction base="itemType">
                        <xs:attribute name = "name" type = "nameType"
 fixed="meat"/>
                </xs:restriction>
        </xs:complexContent>
</xs:complexType>

... and finally I added an attribute "weight":

<xs:complexType name="itemTypeMeatWeight">
        <xs:complexContent>
                <xs:extension base="itemTypeMeat">
                        <xs:attribute name = "weight" type = "xs:int"/>
                </xs:extension>
        </xs:complexContent>
</xs:complexType>

For the "water" is the same as "meat". First I restricted the "name"
attribute to "water":

<xs:complexType name="itemTypeWater">
        <xs:complexContent>
                <xs:restriction base="itemType">
                        <xs:attribute name = "name" type = "nameType"
 fixed="water"/>
                </xs:restriction>
        </xs:complexContent>
</xs:complexType>

... and finally I added the "volume" attribute

<xs:complexType name="itemTypeWaterVolume">
        <xs:complexContent>
                <xs:extension base="itemTypeWater">
                        <xs:attribute name = "volume" type = "xs:int"/>
                </xs:extension>
        </xs:complexContent>
</xs:complexType>

The problem is to define the type for "item" element. If I use
"itemType" as type it the XML does not validated because
"itemType" is abstract. I I use:

<xs:element  name="items">
        <xs:complexType>
                <xs:sequence>
                        <xs:choice>
                                <xs:element name="item"
type="itemTypeWaterVolume"/>
                                <xs:element name="item"
type="itemTypeMeatWeight"/>
                        </xs:choice>
                </xs:sequence>
        </xs:complexType>
</xs:element>

the XML does not validate because the validator assumes that the
"item" type is the first defined (itemTypeWaterVolume). If I
change the declaration of "item"  element then the validator
assumes that the type of "item" is "itemTypeMeatWeight".

Do you know how to solve this problem?

Thanks in advance.

Pau