gistfile1.rb

class Person < ActiveRecord::Base
  has_many :addresses
  validates_associated :addresses
  validates_length_of :addresses, :less_than => 5
  before_save :save_addresses
  
  def addresses_from_form=(address_hashes)
    addresses = address_hashes.collect{ |address_hash| Address.new(address_hash) }
    self.addresses.target = addresses # sets the association target, doesn't trigger any saves or destroys
    # this works because validates_associated basically just does self.addresses.each{ |address| address.valid? }
    # so the net effect is that validation happens on these new, unsaved addresses, so if validation fails, the database is untouched
  end
  
  def save_addresses
    # validation has passed, so it's time to save those addresses
    if addresses.all?{ |address| address.new_record? }
      temp = addresses.to_a
      addresses.reload(true).each{ |address| address.destroy }
      addresses = temp # that should trigger the saving
    end
  end
end