Monkey patching ActiveStorage::Attachment gets lost

Question

So I decided to add an url attr_accessor to ActiveStorage::Attachment objects.

In development the patch holds for a while until it seems to "have been lost". Meaning it works for few minutes, then it does not work anymore. Then I need to restart the server in order to get the patch applied again. I believe I am not patching correctly and I would need advises in that mater.

---

Here is what I tried:

lib/ext/active_storage/attachment.rb

First try :

module ActiveStorageUrl
  extend ActiveSupport::Concern

  included do
    attr_accessor :url
  end
end

ActiveStorage::Attachment.send :include, ActiveStorageUrl

Second try

class ActiveStorage::Attachment < ActiveRecord::Base
  attr_accessor :url
end

---

And by the way in both case it's loaded with this:

config/initializers/monkey_patches.rb

require 'ext/active_storage/attachment'

---

So when it work I have no error message, but after a while the patch "diseapear" (lacking better terms), and I get the following error, telling me my attr_accessor is not there anymore. Rails must have reloaded ActiveStorage classes and my patch is lost.

Module::DelegationError in Products#images
url delegated to blob, but blob is nil

Answer 1

I placed my ActiveStorage::AttachmentMonkeyPatch in /app/models/active_storage/
I've added a callback to get informed if the Attachment has changed. It works fine, all the time.

Maybe this is the issue.

Answer 2

You are probably losing your monkey patch because the code gets reloaded and your ext/active_storage/attachment isn't re-required.

You can tell Rails to run a callback at startup and every time code is reloaded like this.

Rails.configuration.to_prepare do
  require 'ext/active_storage/attachment'
end

Answer 3

Seems to have to do with delegate_missing_to, e.g.

delegate_missing_to :blob

https://github.com/rails/rails/blob/master/activestorage/app/models/active_storage/attachment.rb#L14

Going to where it's defined:

Anyway it might have to do with how attr_accessor works, I would try:

def url
  @url
end

def url=(url)
  @url = url
end

Instead of attr_accessor (which is a C function actually).

Otherwise a really really hacky way to solve this would be to check for ActiveStorage::Attachment.instance_methods.include?(:url) and monkey patch / include / prepend when not present.