Helpers vs. Concerns in Rails

What is a helper?

In Rails, a helper is a method that is used in our Rails views to share reused code across them.

When should I create a helper method?

When we have logic that produces bits of html, such as formatting a string or conditionally rendering page elements, we can consider using a helper method to clean up we code.

How do I write a helper method?

Helpers are modules that fall under app/helpers in our Rails project. Within the module, we can define methods that will be available in all our views.

For example, say we have a User model with attributes first_name and last_name and we'd like to display a user's full name in a view. We could write a helper to re-use:

# app/helpers/user_helper.rb

module UserHelper
  def name_display(user)
    "#{user.first_name user.last_name}"
   end
end

In our view:

# app/views/users/index.haml

%h1 Users Index

%ul
  - User.all.each do |user|
    %li= name_display(user)

Can we use them in controllers?

If we're using Rails 5+, we can use helpers in our controller with the helpers object. Note that it is possible, but it's not super common.

# app/controllers/users_controller.rb

class UsersController < ApplicationController

  def show
   @user = User.find(params[:id])
   @user_name = helpers.name_display(@user)
  end

end

What is a concern?

Concerns are modules that inherit from ActiveSupport::Concern and can be included in controllers or models. The main difference between concerns and regular modules are convenient features such as #included and class_methods. The included block is called whenever a module is “included” into another class or module. It usually includes important class methods / callbacks that we want shared across the models using the concern.

When should I create a concern?

  1. When we have some sort of functionality that is shared across different models. For example, say we have an #archive function which sets archived_at for User and Post. Instead of writing that method in both models, we can write a concern and include Archivable instead.
  2. When we have some sort of functionality that is shared across different controllers. For example, say we are using the pundit gem to handle authorization in an application that enables users to make posts, comment on posts, and like posts. Say we want to skip authorization in actions related to Posts and Comments. We could write a concern that gets included in both controllers.

How do I write a concern?

Concerns live in either app/controllers/concerns or app/models/concerns. Below, I will demonstrate how to write the concerns for the scenarios discussed above.

Model Concerns

Concern: multiple models can be archived by setting the archived_at attribute on the model. We can write an Archivable concern and include it in the necessary models.

The concern:

# app/models/archivable.rb

module Archivable
  extend ActiveSupport::Concern

  included do
    scope :unarchived, -> { where(archived_at: nil) }
    scope :archived, -> { where.not(archived_at: nil) }
  end

  def archive
    self.archived_at = Time.now
    save validate: false
  end

  def archived?
    !archived_at.nil?
  end
end

Including it in User:

# app/models.user.rb

class User < ApplicationRecord
 include Archivable

 ...
end

And again in Post:

# app/models.post.rb

class Post < ApplicationRecord
 include Archivable

 ...
end

Now, we can call the methods defined in the module for instances of User and Post, such as User.last.archive.

Controller Concerns

Concern: multiple controllers should skip authorization (via pundit). Instead of calling the same callbacks twice, we define a concern and include it in the controller.

The concern:

# app/controllers/concerns/skip_authorization.rb

module SkipAuthorization
  extend ActiveSupport::Concern

  included do
    skip_after_action :verify_authorized
    skip_after_action :verify_policy_scoped
  end
end

Including it in our posts_controller:

# app/controllers/posts_controller.rb

class PostsController < ApplicationController
  include SkipAuthorization

  ...
end

And again in comments_controller:

# app/controllers/comments_controller.rb

class CommentsController < ApplicationController
  include SkipAuthorization

  ...
end

What's the difference between a helper and a concern?

To state it simply, helpers contain methods to be used in views. Concerns contain methods to be included in controllers/models.

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