mail_truck.rb

class MailTruck
  attr_accessor :driver, :route
  def initialize( driver, route )
    @driver, @route = driver, route
  end
end

m = MailTruck.new( "Harold", ['12 Corrigan Way', '23 Antler Ave'] )

m.instance_variable_set('@speed', 45)

require 'yaml'

class << m
  def to_yaml_properties
    %w(@driver @route)
  end
end

# or do:
def m.to_yaml_properties
  %w(@driver @route)
end

puts YAML::dump(m)

=begin
Expected output:
--- !ruby/object:MailTruck
driver: Harold
route:
- 12 Corrigan Way
- 23 Antler Ave


Output:
--- !ruby/object:MailTruck
driver: Harold
route:
- 12 Corrigan Way
- 23 Antler Ave
speed: 45
=end