attr_readonlyでレコード作成済みのオブジェクトへの代入でもエラーにする

post.rb

ActiveRecord::ReadonlyAttributes::ClassMethods.module_eval do
  def attr_readonly(*attributes)
    self._attr_readonly = Set.new(attributes.map(&:to_s)) + (_attr_readonly || [])
    attributes.each do |attr|
      define_method attr.to_s + "=" do |text|
        raise NoMethodError.new("NoMethodError: undefined method `#{__method__}' for #{self}", __method__, text) unless new_record?

        super
      end
    end
  end
end

class Post < ApplicationRecord
  attr_readonly :title, :body
end