Re: [xml-dev] An element with more than one possible type

["Radu Cernuta" <radu.cernuta@gmail.com>]

Hello,

XML Schema has something called the Element Declarations Consistent rule that forbids two or more element declarations within the same scope, with the same name and target namespace to have different types. The construct:

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

crosses this rule.

Apart from what Pete said just said, there are no solution for your problem in XML Schema. But you could try to look also at RELAX NG. It can handle your problem elegantly:

<rng:grammar xmlns:rng="http://relaxng.org/ns/structure/1.0"
    datatypeLibrary="http://www.w3.org/2001/XMLSchema-datatypes
">
    <rng:start>
        <rng:element name="items">
            <rng:ref name="itemsContent"/>            
        </rng:element>
    </rng:start>        
    
    <rng:define name="itemsContent">            
        <rng:element name="item">
            <rng:ref name="itemWaterContent"></rng:ref>
        </rng:element>        
        <rng:element name="item">
            <rng:ref name="itemMeatContent"></rng:ref>
        </rng:element>
    </rng:define>
    
    <rng:define name="itemWaterContent">
        <rng:attribute name="name">
            <rng:value>water</rng:value>
        </rng:attribute>
        <rng:attribute name="volume">
            <rng:data type="int"></rng:data>
        </rng:attribute>
    </rng:define>
    
    <rng:define name="itemMeatContent">
        <rng:attribute name="name">
            <rng:value>meat</rng:value>
        </rng:attribute>
        <rng:attribute name="weight">
            <rng:data type="int"></rng:data>
        </rng:attribute>
    </rng:define>
    
</rng:grammar>

I guess this is more or less what you were trying to do in the XML Schema, if only the rules would allow it.

Greetings,

Radu Cernuta

2007/2/2, 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

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