Add additional field to lookup field inside schema.xml

Question

I'm following the steps in this article to setup a joined data view webpart joined on a lookup field:

http://deannaschneider.wordpress.com/2011/11/11/joined-subviews-with-linked-datasources-in-sharepoint-2010/

To summarize these steps, I need to expose the ID column as an additional field in the lookup:

(image copied from linked blog post above)

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I'm creating my lists and lookup columns declaratively inside a Visual Studio solution, and I would like to add this additional field in the xml. How do I do this?

The xml definition for my lookup field is as follows:

<Field ID="{5D184D01-082C-4831-8757-CA2C98F05221}"  
       Name="EventInquiry"  
       DisplayName="Event Inquiry"  
       Type="Lookup"  
       Required="FALSE"  
       Group="Ops Inquiry Tracking Columns"  
       ShowField="EventCode"  
       List="Lists/Event Inquirys"></Field>

Answer 1

To setup the additional field from the lookup, you need to add a second lookup field pointing at the same list, but include the FieldRef attribute on the second lookup field. The value of this attribute should be the same as the ID attribute of the first lookup field.

<Field ID="{393F697A-7F99-488E-B67F-41DFD6D41857}" 
       FieldRef="{5D184D01-082C-4831-8757-CA2C98F05221}"  
       Name="EventInquiryID"  
       DisplayName="Event Inquiry:ID"  
       Type="Lookup"  
       Required="FALSE"  
       ShowField="ID"  
       List="Lists/Event Inquirys"></Field>

Answer 2

For me the example provided on http://sharpointpractice.blogspot.nl/2011/08/sharepoint-field-types-elementsxml.html worked to get the second field from the lookup.