title: loopback: distinct query
body: <p>I am using Loopback to create Rest API. Need to fetch distinct data based on a particular column from a collection. 
  I tried below, but it's not working, instead the below snippet is fetching duplicate data also:</p>
  
  <pre><code>this.app.models.location.find(
                  {
                      distinct: ("regionName",
                      {state: st})
                  }
                  ,
                  function(err, location){........}
  </code></pre>
  
  <p>'RegionName' is the property of 'location' collection and I need data only for selected states (state is another property of location collection) which is represented by 'st'.
  Thanks.</p>