ActiveRecord aggregations and form_for

I'm contemplating about using an ActiveRecord aggregator for some fields.

The thing that bothers me that how well do aggregated attributes work with form_for and input fields. That is, how do you generate the input fields for the aggregated attributes (since they are read only)?

Like, lets take the example from http://api.rubyonrails.org/classes/ActiveRecord/Aggregations/ClassMethods.html .

class Customer < ActiveRecord::Base
    composed_of :balance, :class_name => "Money", :mapping => %w(balance amount)
    composed_of :address, :mapping => [ %w(address_street street), %w(address_city city) ]
  end

class Money
   attr_reader :amount, :currency

   def initialize(amount, currency = "USD")
     @amount, @currency = amount, currency
   end
 end

Now, lets say we had a form, where customer would be allowed to input his own balance. How do you make that f开发者_运维百科orm_for and generate the input fields for balance? Also, where do validations for balance belong to? Does mass assigment work?

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Mass assignment works (through the normal attribute writer methods), but apparently it isn't ran through the aggregator class's initialize method.

Validations also can be normally set on the Customer class.