I'm trying to write a rails application. I'm a ruby-on-rails newbie. I have a User model and a Hobby model. class User < ActiveRecord::Base has_many :hobbies end class Hobby < ActiveRecord::Base belongs_to :user end During the new user registration, I have used a text box where I enter a value for hobby. Then, when I press a button 'Add hobby', a method in UsersController add_hobby is to be called where I intend to append the value entered for hobby by user to the user i.e @user.hobbies << hobby However, my problem is that the user object has not been saved yet, so there is no way to access a particular user object. How do I get around this problem ?