[Rails] belongs_to and callbacks

I have an Order, which belongs to a Buyer. Part of the order#form
allows the user to enter the email address of the buyer. If they email
address doesn't exist in the system, then a Buyer record is created in
a before_save filter.

The weird thing is I have to explicitly set the buyer_id field in a
callback filter. That's telling me that I'm either doing something
wrong, or Rails isn't doing what it should. Here's the sanitized code:

class Order < ActiveRecord::Base
attr_accessor :ordered_by

belongs_to :user
belongs_to :buyer, :class_name => "User", :foreign_key => "buyer_id"

before_save :assign_buyer_by_email

private

def assign_buyer_by_email
self.buyer = User.find_by_email(ordered_by)
if buyer.nil?
self.buyer = User.create(:email => ordered_by)
# snip
self.buyer_id = self.buyer.id # Won't work without this line,
seems un-DRY or worse
end
end
end

ordered_by is the email address on the order#form. You'll notice buyer
shares the table with user. To head off questions about
find_or_create_by, I have other things do to the new user where you
see snip, and it also results in the user being created but the
buyer_id is not set.

Can someone explain what's happening here?

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