Ruby yaml custom domain type does not keep class

Question

I'm trying to dump duration objects (from the ruby-duration gem) to yaml with a custom type, so they are represented in the form hh:mm:ss. I've tried to modify the answer from this question, but when parsing the yaml with YAML.load, a Fixnum is returned instead of a Duration. Interestingly, the Fixnum is the total number of seconds in the duration, so the parsing seems to work, but convert to Fixnum after that.

My code so far:

class Duration
  def to_yaml_type
    "!example.com,2012-06-28/duration"
  end

  def to_yaml(opts = {})
    YAML.quick_emit( nil, opts ) { |out|
      out.scalar( to_yaml_type, to_string_representation, :plain )
    }
  end

  def to_string_representation
    format("%h:%m:%s")
  end

  def Duration.from_string_representation(string_representation)
    split = string_representation.split(":")
    Duration.new(:hours => split[0], :minutes => split[1], :seconds => split[2])
  end
end

YAML::add_domain_type("example.com,2012-06-28", "duration") do |type, val|
  Duration.from_string_representation(val)
end

To clarify, what results I get:

irb> Duration.new(27500).to_yaml
=> "--- !example.com,2012-06-28/duration 7:38:20\n...\n"
irb> YAML.load(Duration.new(27500).to_yaml)
=> 27500
# should be <Duration:0xxxxxxx @seconds=20, @total=27500, @weeks=0, @days=0, @hours=7, @minutes=38>

Answer 1

It look like you’re using the older Syck interface, rather that the newer Psych. Rather than using to_yaml and YAML.quick_emit, you can use encode_with, and instead of add_domain_type use add_tag and init_with. (The documentation for this is pretty poor, the best I can offer is a link to the source).

class Duration
  def to_yaml_type
    "tag:example.com,2012-06-28/duration"
  end

  def encode_with coder
    coder.represent_scalar to_yaml_type, to_string_representation
  end

  def init_with coder
    split = coder.scalar.split ":"
    initialize(:hours => split[0], :minutes => split[1], :seconds => split[2])
  end

  def to_string_representation
    format("%h:%m:%s")
  end

  def Duration.from_string_representation(string_representation)
    split = string_representation.split(":")
    Duration.new(:hours => split[0], :minutes => split[1], :seconds => split[2])
  end
end

YAML.add_tag "tag:example.com,2012-06-28/duration", Duration

p s = YAML.dump(Duration.new(27500))
p YAML.load s

The output from this is:

"--- !<tag:example.com,2012-06-28/duration> 7:38:20\n...\n"
#<Duration:0x00000100e0e0d8 @seconds=20, @total=27500, @weeks=0, @days=0, @hours=7, @minutes=38>

(The reason the result you’re seeing is the total number of seconds in the Duration is because it is being parsed as sexagesimal integer.)