Rails Issue #1585/SO question http://stackoverflow.com/questions/4705867/rails-doesnt-load-classes-on-deserializing-yaml-marshal-objects

my_model.rb

# app/models/my_model.rb

class MyModel
  attr_accessor :id
end

object.yml

--- !ruby/object:MyModel
id: my model

rails console in development environment

$ rails c
Loading development environment (Rails 3.2.3)
1.9.2p290 :001 > object = YAML.load_file "#{Rails.root}/tmp/object.yml"
ArgumentError: undefined class/module MyModel
	from /Users/carol/.rvm/rubies/ruby-1.9.2-p290/lib/ruby/1.9.1/psych/visitors/to_ruby.rb:257:in `path2class'
	from /Users/carol/.rvm/rubies/ruby-1.9.2-p290/lib/ruby/1.9.1/psych/visitors/to_ruby.rb:257:in `resolve_class'
	from /Users/carol/.rvm/rubies/ruby-1.9.2-p290/lib/ruby/1.9.1/psych/visitors/to_ruby.rb:183:in `visit_Psych_Nodes_Mapping'
	from /Users/carol/.rvm/rubies/ruby-1.9.2-p290/lib/ruby/1.9.1/psych/visitors/visitor.rb:7:in `accept'
	from /Users/carol/.rvm/rubies/ruby-1.9.2-p290/lib/ruby/1.9.1/psych/visitors/to_ruby.rb:16:in `accept'
	from /Users/carol/.rvm/rubies/ruby-1.9.2-p290/lib/ruby/1.9.1/psych/nodes/node.rb:25:in `to_ruby'
	from /Users/carol/.rvm/rubies/ruby-1.9.2-p290/lib/ruby/1.9.1/psych.rb:107:in `load'
	from /Users/carol/.rvm/rubies/ruby-1.9.2-p290/lib/ruby/1.9.1/psych.rb:205:in `load_file'
	from (irb):1
	from /Users/carol/.rvm/gems/ruby-1.9.2-p290@railsbug1585/gems/railties-3.2.3/lib/rails/commands/console.rb:47:in `start'
	from /Users/carol/.rvm/gems/ruby-1.9.2-p290@railsbug1585/gems/railties-3.2.3/lib/rails/commands/console.rb:8:in `start'
	from /Users/carol/.rvm/gems/ruby-1.9.2-p290@railsbug1585/gems/railties-3.2.3/lib/rails/commands.rb:41:in `<top (required)>'
	from script/rails:6:in `require'
	from script/rails:6:in `<main>'

rails console in test environment

$ RAILS_ENV=test rails c
Loading test environment (Rails 3.2.3)
1.9.2p290 :001 > object = YAML.load_file "#{Rails.root}/tmp/object.yml"
 => #<MyModel:0x00000102b74dc0 @id="my model"> 

serializing_test.rb

# test/unit/seriralizing_test.rb

require 'test_helper'

class SerializingTest < Test::Unit::TestCase
  def test_yaml_serialize_structure
    my_model = MyModel.new
    my_model.id = 'my model'

    File.open( "#{Rails.root}/tmp/object.yml" , 'w' ) do |f|
      YAML::dump(my_model, f)
    end

    object = YAML.load_file "#{Rails.root}/tmp/object.yml"
    assert( object.instance_of? MyModel ) # passes
    assert_equal( 'my model', object.id ) # passes
  end
end