title: Rails Sunspot + spatial search + group by parent
body: <p>My app have a list of Routes, and each route has a list of Steps.
  I'm using Solr/Sunpost to index the step lat/lng coordinate do do some searches. Piece of code:</p>
  
  <pre><code>class Step &lt; ActiveRecord::Base
     belongs_to :route
     searchable do
          integer :route_id, :references =&gt; Route
          latlon(:coordinates) { Sunspot::Util::Coordinates.new(lat, lng) }
     end
  end
  </code></pre>
  
  <p>I can make searches using the order_by_geodist() from Sunspot, it works ok:</p>
  
  <pre><code>s = Step.search do
     order_by_geodist(:coordinates, lat, lng)
  end
  </code></pre>
  
  <p>The thing is: is there a way to group the results by route_id? In other words, I want to return only one step for each route.</p>