gistfile1.rb

class SubscriptionCharge < ActiveRecord::Base
  belongs_to :subscription
  belongs_to :user

  validate_on_create :payment


  private
    def payment
      if user.charge(amount)  
         subscription.update_attribute(:next_charge_at, 1.month.from_now)
      else
         errors.add(:credit_card, "payment failed")
         #Following is rolled back since validation failed and save runs inside a transaction.
         subscription.update_attributes(:status => "failed", :next_charge_at => 3.days.from_now)
      end
    end
end