# models/user.rb
validates_presence_of :password, :if => :should_validate_password?
validates_presence_of :country, : on => :create
validates_presence_of :state, :if => :in_us?
attr_accessor :updating_password

def in_us?
  country == 'US'
end

def should_validate_password?
  updating_password || new_record?
end

# in controller
@user.updating_password = true
@user.save