var UserSchema = new Schema({
  name: String,
  username: {
    type: String,
    required: [true, "Please enter a username"],
    minlength: [6, "Username must be at least 6 characters"],
    maxlength: [15, "Username cannot exceed 15 characters"],
    unique: true
  },
  password: {
    type: String,
    required: [true, "Please enter a password"],
    minlength: [6, "Password must be at least 6 characters"],
    maxlength: [17, "Password cannot exceed 17 characters"],
  },
  roommates: [{roomate: {type: Schema.Types.ObjectId, ref: "User"}, status: {type: String, default: "pending"}}]
})

update: function(req, res) {
    User.findOne({
      _id: "57f1645c05ec06099ead3db6", 'roommates._id': "57f16436190a09099a1ddbde"
    }, {'roommates.$': 1}, function(err, user1) {
      user1.update()
    })
  }
}