contact.rb

class Contact < ActiveRecord::Base
  has_and_belongs_to_many :clients, class_name: "School"

  after_save :update_primary

  def name
    title + " " + forename + " " + surname
  end

  private
    def update_primary
      if primary
        self.clients.first.contacts.where.not(id: self.id).update_all(primary: false)
      end
    end
end

development.log

  SQL (0.8ms)  INSERT INTO `schools` (`created_at`, `dfes_code`, `key_code`, `local_authority_id`, `name`, `private`, `status`, `updated_at`, `url`) VALUES ('2014-04-28 11:12:46', '0875', 'aaa5', '14', 'aaaaaa', '616161350000', '3. No data', '2014-04-28 11:12:46', 'aaa.cpoms.net')
   (0.3ms)  INSERT INTO `apps_schools` (`school_id`, `app_id`) VALUES (254, 1)
  SQL (0.3ms)  INSERT INTO `contacts` (`created_at`, `email`, `forename`, `notes`, `primary`, `role`, `surname`, `telephone_alternative`, `telephone_primary`, `title`, `updated_at`) VALUES ('2014-04-28 11:12:46', '', 'a', '', 1, '', 'a', '', '', 'a', '2014-04-28 11:12:46')
  School Load (18.0ms)  SELECT `schools`.* FROM `schools` INNER JOIN `contacts_schools` ON `schools`.`id` = `contacts_schools`.`school_id` WHERE `contacts_schools`.`contact_id` = 10 ORDER BY `schools`.`id` ASC LIMIT 1
   (16.0ms)  ROLLBACK
Completed 500 Internal Server Error in 99ms

NoMethodError (undefined method `contacts' for nil:NilClass):
  app/models/contact.rb:13:in `update_primary'