title: Mongo $project _id of a reference collection
body: <p>I have the following document structure:</p>
  
  <pre><code>{
   "_id": ObjectId("52caafa2eccc222244f242cc"),
   "fields": {
       $id: ObjectId("52caafa2eccc222244f242cc"),
       $db: 'other_db',
       $ref: CollectionName 
    }
  }
  </code></pre>
  
  <p>I am trying to project all the {id, fields.id} as following:</p>
  
  <p><code>db.Collection.find({},{"_id": 1, "fields._id": 1});</code>
  but without any result</p>