title: populate multiple sub documents
body: <p>I'm having a user with a group reference. i'm wondering how i populate the game, users and ranks within the group? so what i basically want is to populate these 3 values in the <code>user.group</code> in the code</p>
  
  <p><strong>user mode</strong></p>
  
  <pre><code>var userSchema = new Schema({
    fb: {
      type: SchemaTypes.Long,
      required: true,
      unique: true
    },
    name: String,
    birthday: Date,
    country: String,
    image: String,
    group: { type: Schema.Types.ObjectId, ref: 'Group'}
  
  });
  </code></pre>
  
  <p><strong>group model</strong></p>
  
  <pre><code>var groupSchema = new Schema({
    users: [{
      type: mongoose.Schema.Types.ObjectId,
      ref: 'User'
    }],
    game: { type: Schema.Types.ObjectId, ref: 'Game' },
    ranks: [{
      type: Schema.Types.ObjectId, ref: 'Ladder'
    }]
  
  });
  </code></pre>
  
  <p><strong>code</strong></p>
  
  <pre><code>  User.findByIdAndUpdate(params.id, {$set:{group:object._id}}, {new: true}, function(err, user){
      if(err){
        res.send(err);
      } else {
        res.send(user);
      }
    })
  </code></pre>