search for: zero_to_many

I have a question about regarding the use of ''has_one'' in this scenario: Schema: User ---- id first_name last_name UserMessage ---- user_id message_id Message ---- id Assuming business rules dictate that a Message can have at most ONE User (let''s assume a message can be created without a user associated to it), these are my questions: 1) How would I use has_one