Rails: Set a common or global instance variable across several controller actions

Question

How should one have several different controller\' actions set a common instance variable for use in templates but after the action runs.

Answer 1

I had this challenge when working on a Rails 6 application.

I wanted to use an instance variable in a partial (app/views/shared/_header.html.erb) that was defined in a different controller (app/controllers/categories_controller.rb).

The instance variable that I wanted to use is @categories which is defined as:

# app/controllers/categories_controller.rb

class CategoriesController < ApplicationController

  def index
    @categories = Category.all
  end
  .
  .
  .
end

Here's how I did it:

Firstly, I defined a helper_method for the instance variable in my app/controllers/application_controller.rb file:

class ApplicationController < ActionController::Base
  helper_method :categories

  def categories
    @categories = Category.all
  end
end

This made the @categories instance variable globally available as categories to every controller action and views:

Next,I rendered the app/views/shared/_header.html.erb partial in the app/views/layouts/application.html.erb this way:

<%= render partial: '/shared/header' %>

This also makes the @categories instance variable globally available as categories become available to every controller views that will use the partial without the need to define the @categories instance variable in the respective controllers of the views.

So I used the @categories instance variable globally available as categories in the partial this way:

# app/views/shared/_header.html.erb

<% categories.each do |category| %>
  <%= link_to category do %>
    <%= category.name %>
  <% end %>
<% end %>

Note: You can use locals to pass in the variables into the partials:

<%= render partial: '/shared/header', locals: { categories: @categories } %>

However, this will require a controller action that sets a @categories instance variable for every controller views that will use the partial.

You can read up more about Helpers and Helper Methods in the Rails Official Documentation: https://api.rubyonrails.org/classes/AbstractController/Helpers/ClassMethods.html

That's all.

I hope this helps