validates_associated_with.rb

#propagates association validations to their foreign keys
#rails makes bad assumptions about forms and foreign key validations, so validate the proper way,
#but allow for the ability to propagate validation errors to foreign keys
module ValidatesAssociatedWith
  extend ActiveSupport::Concern

  included do
    class_attribute :associations_validated
    self.associations_validated = []
    before_validation :flush_propagated_association_errors
    after_validation :propagate_association_errors_to_keys
  end

  module ClassMethods
    def validates_associated_with(*args)
      self.associations_validated << args.first
      validates_presence_of *args
    end
  end

  protected
  def purge_propagated_association_errors
    self.each_association_validated do |association|
      errors.delete(self.class.reflect_on_association(association).foreign_key)
    end
  end

  def propagate_association_errors_to_keys
    self.each_association_validated do |association|
      if errors[association].present?
        errors[association].each do |error|
          errors.add(self.class.reflect_on_association(association).foreign_key, error)
        end
      end
    end
  end

  def each_association_validated(&block)
    self.class.associations_validated.each(&block)
  end
end