Relative sitecore query

If a sitecore instance has multiple sites it's quite useful to use relative query for setting a datasource for "picker" fields such as TreeList and TreeListWithQuery.

datasource=$sitename/data

And it works marvelously. But I ran into a case where I had to use a picker on a parameter template and that did not work.

Blog with a solution:

https://jermdavis.wordpress.com/2014/07/14/multilist-fields-with-source-queries-in-parameter-templates/