Provisioning lookup site columns cannot be done through CAML alone. I have seen claims that it could be done (see http://blogs.msdn.com/joshuag/archive/2008/03/14/add-sharepoint-lookup-column-declaratively-through-caml-xml.aspx) but I was never able to make it work when the column to lookup is not “Title” or when it resides on the list in a different SPWeb.
I also saw methdos on the web that suggeest a combination of CAML and an object model but they seem akward and somewhat kludgy (see http://www.sharepointblogs.com/tmt/archive/2007/09/06/sharepoint-programmatically-provisioning-lookup-fields.aspx). Although, if you MUST use CAML for whatever reason they are your only choice. Plus they work :).
As far as I am concerned, if you can avoid CAML and use object model in a feature receiver, you are better off since you get normal debugging and no limitations of CAML.
Here is a little helper method I created that creates a lookup column for you:
public static SPFieldLookup CreateLookupSiteColumn(SPWeb web, string fieldName, Guid lookupListGuid, string lookupFieldName, Guid lookupWebGuid)
{
//create a field and save it
string name = web.Fields.AddLookup(fieldName, lookupListGuid, lookupWebGuid, true);
SPFieldLookup lookup = (SPFieldLookup)web.Fields.GetField(name);
lookup.Group = groupName;
lookup.LookupField = lookupFieldName;
lookup.Update();
web.Update();
return lookup;
}
As always, if you see any mistakes or inaccuracies in this blog, please let me know.
