Rails - build_association is not working for a has_one and belongs_to relationship

Question

I have two models

class Subscription < ActiveRecord::Base
  belongs_to :client
end

class Client < ActiveRecord::Base
  has_one :subscription
end

but when I try to create a parent from the child e.g. sub.build_client the foreign key does not get set e.g.

>> sub = Subscription.new
=> #<Subscription id: nil, token: nil, user_id: nil, created_at: nil, updated_at: nil, cancelled: nil, active: nil, client_id: nil>
>> sub.save(false);
?> client = sub.build_client
=> #<Client id: nil, server_id: nil, ip: nil, created_at: nil, updated_at: nil>
>> client.save(false)
=> true
>> sub.client_id
=> nil
>> sub
=> #<Subscription id: 4, token: nil, user_id: nil, created_at: "2010-01-11 06:07:45", updated_at: "2010-01-11 06:07:45", cancelled: nil, active: nil, client_id: nil>

It does work if I do client.build_subscription

?> client = Client.new
=> #<Client id: nil, server_id: nil, ip: nil, created_at: nil, updated_at: nil>
>> client.save(false)
=> true
>> sub = client.build_subscription
=> #<Subscription id: nil, token: nil, user_id: nil, created_at: nil, updated_at: nil, cancelled: nil, active: nil, client_id: 4>
>> sub.save(false)
=> true
>> sub
=> #<Subscription id: 5, token: nil, user_id: nil, created_at: "2010-01-11 06:09:32", updated_at: "2010-01-11 06:09:32", cancelled: nil, active: nil, client_id: 4>
>> client
=> #<Client id: 4, server_id: nil, ip: nil, created_at: "2010-01-11 06:09:02", updated_at: "2010-01-11 06:09:02">
>> ^C

Ive spent 3 hours fiddling and got nowhere fast. Can anyone explain what I'm doing wrong, things to check etc

Answer 1

According to your model associations, a Subscription is a child of a Client.

If you create a subscription first and then create a client as per your first example, Rails has no way of setting a client_id foreign key value within the subscriptions table because at that point you haven't created the client record, so there's nothing to associate with the subscription. That's why you have to create the parent record (i.e. a client) first and then associate it with a child subscription record using the build_subscription method.