Using virtual attributes for multi-parameter form helpers in Rails

In a Rails application I am working on, I needed to setup a form with a field with a non-standard behaviour. The field represents a date object, so the FormHelper date_select helper looked great; however, the date to display was not the actual date to be set on the database, but the day before. Changing all the data on the DB was a bit risky, so I had to stick with this requirement.
I decided to use a virtual attribute to do this, as it seemed the most elegant solution, so I wrote this in my model:

	def expire_date_minus_one_day
	self.expire_date - 1.day
	end
	def expire_date_minus_one_day= date
	self.expire_date = date + 1.day
	end

view raw user_subscription.rb hosted with ❤ by GitHub

And this in my controller:

	<p>
	<%= f.label :expire_date %>
	<%= f.date_select :expire_date_minus_one_day %>
	</p>

view raw _form_for_time.html.erb hosted with ❤ by GitHub

However, trying to send this data to the controller resulted in a "1 error(s) on assignment of multiparameter attributes" error.

The solution I found after some search was this one:

	composed_of :expire_date_minus_one_day,
	:class_name => 'Date',
	:mapping => %w(Date to_s),
	:constructor => Proc.new{ |item| item },
	:converter => Proc.new{ |item| item }

view raw user_subscription.rb hosted with ❤ by GitHub

Reference:
http://gabeodess.heroku.com/posts/14