I would like to map a relation between two Rails models, where one side can be optionnal. Let's me be more precise...

I have two models: Profile that stores user profile information (name, age,...) and User model that stores user access to the application (email, password,...).

To give you more information, User model is handled by Devise gem for signup/signin.

Here is the scenario of my app:

1/ When a user register, a new row is created in User table and there is an equivalent in Profile table. This leads to the following script:

class User < ActiveRecord::Base
    belongs_to :profile
end

2/ A user can create it's profile without registering (kind of public profile with public information), so a row in Profile doesn't have necessarily a User row equivalent (here is the optional relation, the 0..1 relation in UML).

Question: What is the corresponding script to put in class Profile < AR::Base to map optionally with User?

Thanks in advance.