Having a debate about better coding practices is a healthy part of growth for any developer, especially within opinionated frameworks like Ruby on Rails. One feature that often sparks discussion is the use of ActiveSupport::Concern.

To illustrate, 37signals recently released Writebook, showcasing effective use of controller and model concerns. They adhered to Rails conventions without creating different folder structures or combining related classes in one file to minimize file count. Instead, they followed the Rails doctrine of convention over configuration. While it’s not mandatory to mimic their approach, having a consistent convention in place is beneficial, allowing flexibility in how you implement it.

The debate about using ActiveSupport::Concern is not new, and it won't be the last. Opinions on its use often come from personal preferences or company policies rather than understanding why Rails includes it and its merits or drawbacks.

While I set to do further reading about it, to form my own reasoning, I received an email from Hrishi Mittal's Learnetto newsletter. He sent out a quiz for Rails developer and the first question was about concern, what a coincidence.

- What is the purpose of the `ActiveSupport::Concern` module in Rails?

1.  To handle HTTP requests and responses
2.  To manage database migrations
3.  To define model associations
4. To provide a way to extend classes with reusable functionality

module Commentable
  extend ActiveSupport::Concern

  included do
    # Class-level methods go here
  end

  def comments_count
    # Instance method implementation
  end
end

class Post < ApplicationRecord
  include Commentable
end

module Logger
  extend ActiveSupport::Concern

  included do
    before_action :log_action
  end

  private

  def log_action
    Rails.logger.info("Action #{action_name} performed on #{model_name.humanize.downcase}")
  end
end

class Post < ApplicationRecord
  include Logger
end

# Creating a post and performing actions on it will now be logged.
post = Post.create!(title: "Ruby on Rails Tutorial", body: "Learn Ruby on Rails the easy way.")
post.update(body: "Updated content.") # This action will be logged.

module CommentAttributes
  extend ActiveSupport::Concern

  included do
    attribute :content, :string
    attribute :user_id, :integer
  end
end

module Commentable
  extend ActiveSupport::Concern

  included do
    has_many :comments
    include CommentAttributes
  end
end

class Post < ApplicationRecord
  include Commentable
end

# Now, when creating a comment, we can directly assign values to the attributes defined in CommentAttributes.
comment = post.comments.create!(content: "Great tutorial!", user_id: current_user.id)

module CommentCounter
  extend ActiveSupport::Concern

  included do
    has_many :comments
  end

  def comments_count
    comments.count
  end
end

class Post < ApplicationRecord
  include CommentCounter
end

# Now, we can easily get the total number of comments on a post.
post = Post.create!(title: "Ruby on Rails Tutorial", body: "Learn Ruby on Rails the easy way.")
post.comments.create!(content: "Great tutorial!") # Add a comment to the post.
puts "Total comments on this post: #{post.comments_count}"

class Post < ApplicationRecord
  include Commentable
  include Searchable
end