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class Product < ApplicationRecord
  has_many :orders

  after_update do
    if price_changed?
      orders.each do |order|
        order.update! price: price
      end
    end
  end
end

# ./app/subscribers/order_subscriber.rb
subscribe_model :product, :after_update do
  if price_changed?
    orders.each do |order|
      order.update! price: price
    end
  end
end

# ./config/initializers/subscribe_model.rb
def subscribe_model(model_name, event_name, &block)
  ActiveSupport::Notifications.subscribe("active_record.#{model_name}.#{event_name}") do |_name, _started, _finished, _unique_id, data|
    data[:model].instance_eval(&block)
  end
end

class ActiveRecord::Base
  class_attribute :skip_model_subscribers
  self.skip_model_subscribers = false
end

%i(after_create after_update after_destroy after_save after_commit after_create_commit after_update_commit).each do |name|
  ActiveRecord::Base.public_send(name) do
    unless skip_model_subscribers
      readonly! unless readonly?
      ActiveSupport::Notifications.instrument "active_record.#{self.class.model_name.singular}.#{name}", model: self
      ActiveSupport::Notifications.instrument "active_record.#{self.class.base_class.model_name.singular}.#{name}", model: self if self.class.base_class != self.class
      public_send(:instance_variable_set, :@readonly, false)
    end
  end
end

Rails.application.config.after_initialize do
  Dir[Rails.root.join('app/subscribers/*.rb')].each { |f| require f }
end