def method_missing(method_name, *args, &block)
    if method_name =~ /^swap_(.+)_with(!?)$/
      property = $1
      is_bang  = $2 == '!'
      sibling  = args[0]
      #errors
      errors = {
        :no_response => 'does not respond to "%s"' % property,
        :bang => 'Can not execute bang method on objects that do not respond to "save" method'
      }

      raise '%s %s' % [self.class.name, errors[:no_response]] unless self.respond_to?(property)
      raise '%s %s' % [sibling.class.name, errors[:no_response]] unless sibling.respond_to?(property)
      raise errors[:bang] if is_bang && (!self.respond_to?('save') || !sibling.respond_to?('save'))

      old_sibling_property = sibling.send(property)

      sibling.send("#{property}=", self.send(property))
      self.send("#{property}=", old_sibling_property)

      [self, sibling].collect(&:save) if is_bang
      return self
    end
  end