title: Mongoose populate sub-sub document
body: <p>I have this setup in my MongoDB</p>
  
  <p>Items:</p>
  
  <pre><code>title: String
  comments: [] // of objectId's
  </code></pre>
  
  <p>Comments:</p>
  
  <pre><code>user: ObjectId()
  item: ObjectId()
  comment: String
  </code></pre>
  
  <p>Here's my Mongoose schema:</p>
  
  <pre><code>itemSchema = mongoose.Schema({
      title: String,
      comments: [{ type: Schema.Types.ObjectId, ref: 'comments' }],
  });
  
  Item = mongoose.model('items', itemSchema);
  
  commentSchema = mongoose.Schema({
      comment: String,
      user: { type: Schema.Types.ObjectId, ref: 'users' },
  });
  
  Comment = mongoose.model('comments', commentSchema);
  </code></pre>
  
  <p>This is where I get my items along with the comments:</p>
  
  <pre><code>Item.find({}).populate('comments').exec(function(err, data){
      if (err) return handleError(err);
      res.json(data);
  });
  </code></pre>
  
  <p>How do I populate the comments array with it's respective user? Since each comment has a user ObjectId()?</p>