Validating Existence of a Nested Model in Rails

I'm a big fan of Nested Attributes in Rails. It makes my head hurt less, and makes my forms look better. I did have trouble wrapping my head around one aspect of Nested Attributes through, and that was ensuring that a user could not have less than X associated models after a save or update.

#app/models/user.rb
class User < ActiveRecord::Base
 has_many :addresses
 accepts_nested_attributes_for :addresses, :allow_destroy => true, :reject_if => lambda {|a| a[:city].blank?}
end

#app/models/address.rb
class Address < ActiveRecord::Base
 belongs_to :user
end

We know that the user model needs to have at least one address. How do you test for that though? If the user selects to mark all the records for deletion without creating a new one, we're screwed. Digging through the API I found the "marked_for_destruction?" method, and the answer to my question.

#app/models/user.rb
class User < ActiveRecord::Base
 has_many :addresses
 accepts_nested_attributes_for :addresses, :allow_destroy => true, :reject_if => lambda {|a| a[:city].blank?}

 validate :at_least_one_address

 private

 def at_least_one_address
  delCount = 0
  self.addresses.each {|a| delCount += 1 if a.marked_for_destruction?} #iterate through each record and check if it's going to be deleted
  errors.add_to_base "You must have at least one address entered." if delCount >= self.addresses.length #if our number of records about to be deleted is greater than or equal to our number of addresses, we raise an error
 end
end

As always, I hope this saves someone some time!