has_one

A has_one association sets up a one-to-one connection with another model, but with different semantics. This association indicates that each instance of a model contains or possesses one instance of another model.

For example, if each user in your application has only one account, you'd declare the user model like this:

class User < ApplicationRecord
  has_one :account
end

In Active Record, when you have a has_one relation, active record ensures that the only one record exists with the foreign key.

Here in our example: In accounts table, there can only be one record with a particular user_id. If you try to associate one more account for the same user, it makes the previous entry's foreign key as null(making it orphan) and creates a new one automatically. It makes the previous entry null even if the save fails for the new entry to maintain consistency.

user = User.first
user.build_account(name: "sample")
user.save   [Saves it successfully, and creates an entry in accounts table with user_id 1]
user.build_account(name: "sample1")  [automatically makes the previous entry's foreign key null]
user.save  [creates the new account with name sample 1 and user_id 1]