Combine (compute) fields in XForms itemset

Question

I have an instance with a list of artists

<xf:instance id="Artists" xmlns="">  
    <Artists>
        <Artist ID="6548">
            <FirstName>Elvis</FirstName>
            <LastName>Presley</LastName>
        </Artist>
        <Artist ID="7895">
            <FirstName>Duane</FirstName>
            <LastName>Eddy</LastName>
        </Artist>
    </Artists>
</xf:instance>    

And I want to select Artists from the list. This code:

<xf:select>
    <xf:label>Artists</xf:label>
    <xf:itemset nodeset="instance('Artists')/Artist">
        <xf:label ref="LastName" ></xf:label>
        <xf:value ref="@ID"></xf:value>
    </xf:itemset>
</xf:select>

Shows "Presley" and "Eddy" labels, but I would like to show "Presley, Elvis" and "Eddy, Duane". Can this be done without modifying the model? I suppose there is some way to put an expression there, but I don't know.

I tried:

<xf:label ref="concat(LastName, ',', FirstName)" ></xf:label>

But that just does not show anything. (I am using BetterForm and eXist)

Answer 1

This syntax works in BetterForm+eXist:

<xf:label>
    <xf:output value="concat(LastName, ',', FirstName)" />
</xf:label>

Answer 2

Try this:

<xf:label value="concat(LastName, ', ', FirstName)" />

This solution may be implementation-specific, i've tested it only with orbeon forms. The sole difference to your code is the value attribute in the xf:label element instead of @ref. ref="concat(LastName, ',', FirstName)" won't work since with ref, you specify a singe-node binding. If there's no such node in the model, it will fail. With value instead, you can create a "computed" label value, using XPath.