not rewritable validator

en.yml

en:
  errors:
    messages:
      changed: "can't be changed"

not_rewritable_validator.rb

class NotRewritableValidator < ActiveModel::EachValidator
  def validate_each(record, attribute, value)
    return unless record.changed.include? attribute.to_s
    return if record.changes[attribute.to_s].first.blank?

    record.restore_attributes([attribute])
    record.errors.add(attribute, options[:message] || I18n.t('errors.messages.changed'))
  end
end

not_rewritable_validator_spec.rb

describe NotRewritableValidator do
  subject do
    Class.new do
      include ActiveModel::Validations
      include ActiveModel::Dirty
      validates :write_once, not_rewritable: true

      define_attribute_methods :write_once

      def initialize
        @write_once = nil
      end

      attr_reader :write_once

      def write_once=(val)
        write_once_will_change! unless val == @write_once
        @write_once = val
      end

      def save
        # do persistence work
        changes_applied
      end

      def reload!
        # get the values from the persistence layer
        clear_changes_information
      end

      def rollback!
        restore_attributes
      end
    end.new
  end

  describe 'nil field' do
    it { expect(subject.write_once).to be nil }

    it 'can be changed' do
      subject.write_once = 'something'
      subject.validate
      expect(subject.errors[:write_once]).not_to include "can't be changed"
    end

    it 'does not revert the change' do
      expect {
        subject.write_once = 'something'
        subject.validate
        subject.save
      }.to change {
        subject.write_once
      }.from(nil).to('something')
    end
  end

  describe 'blank field' do
    before do
      subject.write_once = ''
      subject.validate
      subject.save
    end

    it { expect(subject.write_once).to eq('') }

    it 'can be changed' do
      subject.write_once = 'something'
      subject.validate
      expect(subject.errors[:write_once]).not_to include "can't be changed"
    end

    it 'does not revert the change' do
      expect {
        subject.write_once = 'something'
        subject.validate
        subject.save
      }.to change {
        subject.write_once
      }.from('').to('something')
    end
  end

  describe 'once field has been set' do
    before do
      subject.write_once = 'a thing'
      subject.validate
      subject.save
    end

    it 'makes the record invalid if changed' do
      subject.write_once = 'another thing'
      subject.validate
      expect(subject.errors[:write_once]).to include "can't be changed"
    end

    it 'reverts the change' do
      expect {
        subject.write_once = 'another thing'
        subject.validate
        subject.save
      }.not_to change {
        subject.write_once
      }.from('a thing')
    end
  end
end

I realize rolling back the change is a side effect and some might say it's a bad idea. I like it.