Has_one behavior - AWDWR page 332


On AWDWR page 332 there is this example

class Order < ActiveRecord::Base
has_one :invoice

order = Order.new(… attributes …)
invoice = Invoice.new(… attributes …)
order.invoice = invoice

And then the explanation: If there is already an existing child object
when you assign a new object to a has_one association, that existing
object will be updated to remove its foreign key association with the
parent row(the foreign key will be set to null)

I find that however that if:

  1. I change has_one :invoice :dependent => :destroy then the existing
    object is actually DELETED rather than just FK being set to null

  2. However for the same has_one :invoice :dependent => :destroy if I
    use build_invoice instead of assignment
    i.e. order.build_invoice instead of order.invoice = invoice
    then the Existing object’s FK is NULLED out and NOT deleted.

I’m just trying to figure if:

a) Both observations above are correct
b) Why the difference in behavior - i.e. assignment DELETEs the
existing record, whereas build_association NULLs the FK