# Safely extend a Ruby on Rails controller

With Avo, being a Ruby on Rails engine, we have a few controllers where we "chew up" the request, fetch the data, and display the results. This includes our own `ApplicationController`.

Over the weekend, one of our customers reported an issue after they updated to Avo 2.17. After a bit of digging I figured that one helper we introduced in the `ApplicationController` was missing from their setup. Fear came over me; "did we forget to add that helper?", "Was it somehow removed in a merge?", "is this happening to everyone?".
I started digging in. I opened the gem using `bundle open avo` and checked. We didn't forget about it. It was there.

Hmmm. Weird. I started to think 🤔 "could it be? could they have overriden the `ApplicationController` class?". Yes they did! They did it ot add a concern that deals with some multitenancy scenario.

```ruby
# Copied from Avo to `app/controllers/avo/application_controller.rb`
module Avo
  class ApplicationController < ::ActionController::Base
    include Pagy::Backend
    include Avo::ApplicationHelper
    include Avo::UrlHelpers

    protect_from_forgery with: :exception
    around_action :set_avo_locale
    before_action :multitenancy_detector

    # ... more Avo::ApplicationController methods

    def multitenancy_detector
      # your logic here
    end
  end
end
```

That fixed their business need but introduced a a very fragile piece of code that broke as soon as we updated the application controller.
Fortunately there's a nice way to fix that.

### Create a concern that holdes your piece of code.

```ruby
# app/controllers/concerns/multitenancy.rb
module Multitenancy
  extend ActiveSupport::Concern

  included do
    before_action :multitenancy_detector
  end

  def multitenancy_detector
    # your logic here
  end
end
```

### Include it in the controller from the outside

```ruby
# configuration/initializers/avo.rb
Rails.configuration.to_prepare do
  Avo::ApplicationController.include Multitenancy
end
```

Now, the concern, the `multitenancy_detector` method, and the `before_action` are safely included in `Avo::ApplicationController`.

More on this on our docs about the [ApplicationController](https://docs.avohq.io/3.0/avo-application-controller.html) ✌️

#### Bonus

If you'd like to add a `before_action` before all of Avo's before actions, use `prepend_before_action` instead. That will run that code first and enable you to set an account or do something early on.