Ran into a problem today where I have a class with a few attributes on it, but I only want a certain three of those attributes to appear in the YAML dump of a class instance.

Diving straight into a code example–lets say we have a Contact class, and we only want to dump the name, email and website attributes.

require "yaml"

class Contact
  attr_accessor :name, :email, :website, :telephone

  # helper method to make setting up easy
  def initialize(params={})
    params.each do |key, value|
      meffod = "#{key.to_s}="
      send(meffod, value) if respond_to?(meffod)
    end
  end
end

# And create an instance for us to play with
caius = Contact.new(
  :name => "Caius",
  :email => "dev@caius.name",
  :website => "http://caius.name/",
  :telephone => "12345"
)

As we’d expect when dumping this, all instance variables get dumped out:

print caius.to_yaml
# >> --- !ruby/object:Contact 
# >> email: dev@caius.name
# >> name: Caius
# >> telephone: "12345"
# >> website: http://caius.name/

Initially I tried to override to_yaml and unset the instance variables I didn’t want showing up, but that just made them show up empty. After digging around a bit more, I happened across the Type Families page in the yaml4r docs, which right at the bottom mentions to_yaml_properties.

Turns out to_yaml_properties returns an array of instance variable names (as strings) that should be dumped out as part of the object. A quick method definition later, and we’re only dumping the variables we want. (See my Ruby Shortcuts post if you don’t know what %w() does)

class Contact
  def to_yaml_properties
    %w(@name @email @website)
  end
end

And now we dump the class, expecting only the three attributes to be outputted:

print caius.to_yaml
# >> --- !ruby/object:Contact 
# >> name: Caius
# >> email: dev@caius.name
# >> website: http://caius.name/

Success!