title: Meteor mongoDB query on a selector but unique results based on another
body: <p>Is there any way to query on a field but make sure the result is unique based on another field ? </p>
  
  <p>ex:</p>
  
  <pre><code>Available.find({available: "2016-02-06"}, {sort: {userId: 1}}; 
  </code></pre>
  
  <p>Now I want to make sure there are no duplicate userId's in result. I could'nt find anything built in, what is the best way to achieve this ?</p>
  
  <p>*I know I can use _.uniq but not sure if this is efficient on a larger selection?</p>