Set an instance variable app wide in Rails

Question

In my Rails application I have a class that I want to initialize and then access it throughout my controllers. So the idea is that I set it via the application controller if it's not already been defined:

class ApplicationController < ActionController::Base

  before_action :set_custom_class

  # create an instance of customclass if doesn't exist
  def set_custom_class
    @custom_class ||= CustomClass.new
  end

end

An example of the class:

class CustomClass

  def initialize; end

  def custom_method
    @custom_method
  end

  def custom_method=(content)
    @custom_method = content
  end

end

If I then have a controller like:

class MyController < ApplicationController

  def method_1
    # set the custom_method value on my instance
    @custom_class.custom_method('Some content')
    # return the value I set above
    @variable = @custom_class.custom_method
    redirect_to :method_2
  end

  def method_2
    # I should be able to retrieve the same value from that same instance
    @variable = @custom_class.custom_method
  end

end

What I'm finding is that when calling method_1 the @variable will return my content fine, but when calling method_2 AFTER method_1 (so the custom_method for the app wide @custom_class has been set) it's returning nil.

Why isn't the instance being retained? The @custom_class shouldn't be creating a new instance as it's already been set. So I can't understand why the value I have set gets lost when requesting it.

Can you check if `set_custom_class` is creating a new object every time or is it really caching? maybe the issue is there — amrdruid, Jul 17 '18 at 19:58

score 2 · Answer 1 · answered Jul 17 '18 at 19:59

2

You witnessing such behaviour, because state of a controller is not preserved between requests. For example, imagine that current_user method sets @current_user for one request and returns the same user for another one.

Please, consider an option of using cookies or database for sharing state between requests.

Otherwise, a workaround would be setting a class variable of CustomClass, but I don't recommend to do it.

answered Jul 17 '18 at 19:59

Igor Drozdov

14,690
5
37
53

Thanks for that. I was under the impression that once you created an instance in the application controller it would be retained but now that I think about it... why would it! The reason I asked was I was looking into using this Gem: https://github.com/zokioki/fitbit_api/blob/master/lib/fitbit_api/client.rb#L51 and this line is where the same issue happens in that when you create a client and get a token back you lose it the very next request making that method useless unless you keep sending a refresh token to a new instance. – Cameron Jul 17 '18 at 20:27
I've even raised a ticket for it: https://github.com/zokioki/fitbit_api/issues/4 as it seems that's a bit weird to have that method outside of using the refresh_token version of initializing the client. – Cameron Jul 17 '18 at 20:28

score 0 · Answer 2 · answered Jul 17 '18 at 19:59

0

Looks like your before_action will re-instantiate the new object on every request. That means that since you aren't passing anything through to the class in Method2, it will come out as NULL.

answered Jul 17 '18 at 19:59

Kao

11
2

score -1 · Answer 3 · answered Jul 17 '18 at 21:51

Since you said app-wide, why not make it app-wide?

In config/application.rb,

module App
  class Application < Rails::Application

    def custom_class
      @custom_class ||= CustomClass.new
    end 
  end 
end

in your application code,

Rails.application.custom_class

Set an instance variable app wide in Rails

3 Answers3