Rails validation design question

gistfile1.rb

class Contribution < ActiveRecord::Base
  attr_accessible :reward_id, :reward
  belongs_to :reward

  validates_presence_of :amount
  validates_numericality_of :amount, greater_than: 0
  validates_presence_of :reward

  validates_each :amount, unless: Proc.new{ |contrib| contrib.reward.nil? } do |contribution, attr, value|
    reward = contribution.reward
    contribution.errors.add(attr,
                            "Contribution must be at least #{reward.minimum_contribution} #{reward}'s minimum contribution") unless value > reward.minimum_contribution
  end

end