Concern trong Rails là gì và dùng trong Controller/Model thế nào?

Concern là module được include vào nhiều class để tái sử dụng logic.

Rails cung cấp ActiveSupport::Concern để tránh boilerplate.

# app/controllers/concerns/authenticatable.rb
module Authenticatable
  extend ActiveSupport::Concern

  included do
    before_action :require_login
  end

  def require_login
    redirect_to login_path unless current_user
  end
end

class PostsController < ApplicationController
  include Authenticatable
end

Model Concern hoạt động tương tự — thường dùng để share scope, callback, hay helper method giữa nhiều model.

Concern tốt cho logic dùng ở nhiều nơi, nhưng không nên lạm dụng để "giấu" logic phức tạp.

A Concern is a module included in multiple classes to share logic.

Rails provides ActiveSupport::Concern to reduce boilerplate.

# app/controllers/concerns/authenticatable.rb
module Authenticatable
  extend ActiveSupport::Concern

  included do
    before_action :require_login
  end

  def require_login
    redirect_to login_path unless current_user
  end
end

class PostsController < ApplicationController
  include Authenticatable
end

Model Concerns work the same way — commonly used to share scopes, callbacks, or helper methods across models.

Concerns are useful for logic shared in many places, but shouldn't be overused to "hide" complex logic.