Using ActiveRecord with DelegateClass - ruby

Question

Using rails 3.2.8 I have a model setup with a couple attributes
class MyModel < ActiveRecord::Base
attr_accessible :foo, :bar
end
I have another class setup using the above as a delegate class
class MyModelPresenter < DelegateClass(MyModel)
def initialize(month, obj)
#month = month
super(obj)
end
def self.build(month, attributes = { })
new(month, MyModel.new).tap do |p|
p.attributes = attributes
end
end
def attributes=(attributes)
attributes.each { |k, v| send("#{k}=", v) }
end
end
When I create a new MyModelPresenter like so:
MyModelPresenter.build(Date.today, {:foo => 1})
I get the following back
NoMethodError: undefined method `foo=' for #<MyModel:0x1098f31a8>
from /Users/me/.rbenv/versions/ree-1.8.7-2011.03/lib/ruby/gems/1.8/gems/activemodel-3.2.8/lib/active_model/attribute_methods.rb:404:in `method_missing'
from /Users/me/.rbenv/versions/ree-1.8.7-2011.03/lib/ruby/gems/1.8/gems/activerecord-3.2.8/lib/active_record/attribute_methods.rb:149:in `method_missing'
from /Users/me/dev/temp/app/presenters/my_model_presenter.rb:23:in `send'
from /Users/me/dev/temp/app/presenters/my_model_presenter.rb:23:in `attributes='
from /Users/me/dev/temp/app/presenters/my_model_presenter.rb:23:in `each'
from /Users/me/dev/temp/app/presenters/my_model_presenter.rb:23:in `attributes='
from /Users/me/dev/temp/app/presenters/my_model_presenter.rb:17:in `build'
from /Users/me/dev/temp/app/presenters/my_model_presenter.rb:15:in `tap'
from /Users/me/dev/temp/app/presenters/my_model_presenter.rb:15:in `build'
For some reason the database attributes on the model aren't getting defined (setters or getters). Before upgrading to rails 3.2 this was all working in a rails 3.1 app.
Does anyone have any idea why the model's attribute methods aren't getting defined?

Answer 1

Inheriting from Delegator instead of using DelegateClass solved the problem. Here is what the final class looked like:
class MyModelPresenter < Delegator
def initialize(month, obj)
#month = month
super(obj)
#_sd_obj = obj
end
def __getobj__
#_sd_obj
end
def self.build(month, attributes = { })
new(month, MyModel.new).tap do |p|
p.attributes = attributes
end
end
def attributes=(attributes)
attributes.each { |k, v| send("#{k}=", v) }
end
end
As you can see I also had to add the def __getobj__ method. and set #_sd_obj in the initializer to an instance of the class I'm delegating from.
See the SimpleDelegator example.