title: mongodb java driver. How to return object by reference
body: <p>For example, i have two collections "animals" and "food".
  In collection "animals" there is a field food, which is reference to collection "food" (not embedded).
  Im using mongo java driver:</p>
  
  <pre><code>    DBCollection collection = db.getCollection("animals");
      DBObject fields = new BasicDBObject("name", 1);
      fields.put("food", 1);
      fields.put("_id", 0);
      DBObject project = new BasicDBObject("$project", fields );
      collection.aggregate(project);
  </code></pre>
  
  <p>it return name and Id of object "food". 
  So what would be the proper way to get fields of object "food"? </p>