title: Reference is working like Embed while Associating
body: <p>I was learning Associations in MongoDB. There I learned how to use Embed, but while working on References, I got stuck for few days as its working same as Embed. Instead of showing just the ObjectId in the array, the whole object is being added to the array.
  Here's the snippet,</p>
  
  <p>campground schema:</p>
  
  <pre><code>var campgroundSchema = new mongoose.Schema({
    name: String,
    image: String,
    description: String,
    comments: [
      {
          type: mongoose.Schema.Types.ObjectId,
          ref: "Comment"
      }
  ]
  });
  </code></pre>
  
  <p>comment schema:</p>
  
  <pre><code>var commentSchema = mongoose.Schema({
    text: String,
    author: String
  });
  </code></pre>
  
  <p>Models</p>
  
  <pre><code>var Campground = mongoose.model("Campground", campgroundSchema);
  var Comment = mongoose.model("Comment", commentSchema);
  </code></pre>
  
  <p>Output:</p>
  
  <pre><code>{ "_id" : ObjectId("5a71970ec0379521607b5f47"), "comments" : ***[ { "_id" : 
  ObjectId("5a71970ec0379521607b5f4a"), "text" : "this place is gr8", "author" 
  : "Homer", "__v" : 0 } ]***, "name" : "Clould's Rest", "image" : 
  "https://farm7.staticflickr.com/6014/6015893151_044a2af184.jpg", 
  "description" : "blah blah", "__v" : 1 }
  </code></pre>
  
  <p>what I mean to say is instead of showing the whole object inside comments array, the array must show only the reference i.e ObjectId
  like this</p>
  
  <pre><code>{ "_id" : ObjectId("5a71970ec0379521607b5f47"), "comments" 
  : ObjectId("5a71970ec0379521607b5f4a") ], "name" : "Clould's Rest", "image" : 
  "https://farm7.staticflickr.com/6014/6015893151_044a2af184.jpg", 
  "description" : "blah blah", "__v" : 1 }
  </code></pre>
  
  <p>that's how references work right?</p>