Adding #to_yaml to DataMapper models

Question

I am using DataMapper for Database access. My goal is to send the models to an webservice as read-only object. This is my current try:

class User
  include DataMapper::Resource

  def to_yaml(opts = {})
    mini_me = OpenStruct.new
    instance_variables.each do |var|
      next if /^@_/ =~ var.to_s
      mini_me.send("#{var.to_s.gsub(/^@/, '')}=", instance_variable_get(var))
    end

    mini_me.to_yaml(opts)
  end

  ....
end

YAML::ENGINE.yamler = 'psych'

u = User.get("hulk")
p u.to_yaml
# => "--- !ruby/object:OpenStruct\ntable:\n  :uid: hulk\n  :uidNumber: 1000\n  :gidNumber: 1001\n  :email: hulk@example.com\n  :dn: uid=hulk,ou=People,o=example\n  :name: Hulk\n  :displayName: Hulk\n  :description: Hulk\n  :homeDirectory: /home/hulk\n  :accountFlags: ! '[U          ]'\n  :sambaSID: S-1-5-21-......\nmodifiable: true\n" 

p [ u ].to_yaml # TypeError: can't dump anonymous class Class

Any ideas how to make this work and get rid of the exception?

Thanks, krissi

Answer 1

Using to_yaml is deprecated in Psych, and from my testing it seems to be actually broken in cases like this.

When you call to_yaml directly on your object, your method gets called and you get the result you expect. When you call it on the array containing your object, Psych serializes it but doesn’t correctly handle your to_yaml method, and ends up falling back onto the default serialization. In your case this results in an attempt to serialize an anonymous Class which causes the error.

To fix this, you should use the encode_with method instead. If it’s important that the serialized form is tagged as an OpenStruct object in the generated yaml you can use the represent_object (that first nil parameter doesn’t seem to be used):

def encode_with(coder)
  mini_me = OpenStruct.new
  instance_variables.each do |var|
    next if /^@_/ =~ var.to_s
    mini_me.send("#{var.to_s.gsub(/^@/, '')}=", instance_variable_get(var))
  end

  coder.represent_object(nil, mini_me)
end

If you were just using OpenStruct for convenience, an alternative could be something like:

def encode_with(coder)
  instance_variables.each do |var|
    next if /^@_/ =~ var.to_s
    coder[var.to_s.gsub(/^@/, '')]= instance_variable_get(var)
  end
end

Note that Datamapper has its own serializer plugin that provides yaml serialization for models, it might be worth looking into.