title: Thinking Sphinx: Querying only specific fields in a models index
body: <p>Lots of Thinking Sphinx questions from me lately. Here's another one:</p>
  
  <p>In some of my searches I'd like to only query some of all the fields included in my index definition. I can't seem to find any on this, the nearest thing being limiting the global search to specific models.</p>
  
  <p>So.. is it possible to query only a given subset of fields in an index?</p>