:validate => true on a belongs_to

:validate => true on a belongs_to tells Rails to validate the owner
record when the owner record is being saved in order to get a foreign
key value for the association. This doesn't seem to make sense,
because you cannot save an association without saving first the owning
record, so let's say:

class Address < ActiveRecord::Base
  belongs_to :user, :validate => true
end

class User < ActiveRecord::BAse
  has_many :addresses
end

u = User.new
a = Address.new
u.addresses << a

u.save #I guess this is the only time when the belongs_to :validate =>
true will be fired, that is, when we are saving the owner record. But
this doesn't make sense because the owner record's own validation
would be triggered anyway when we do this. So I still don't understand
the purpose of :validate => true on a belongs_to relation.

thanks for response