A controller that can not be extended without being modified

application_controller.rb

class ApplicationController < ActionController::Base
  before_filter :load_user
    
  def load_user
    @user ||= User.find(params[:user_id])
  end
end

events_controller.rb

class EventsController < ApplicationController
  def index
    render json: @user.events, each_serializer: EventsSerializer
  end
end

RegistrationsController

class RegistrationsController < ApplicationController
  def create
    # Some logic to sign up
    render json: @user, each_serializer: UserSerializer
  end
end

tasks_controller.rb

class TasksController < ApplicationController
  def index
    render json: @user.tasks, each_serializer: TasksSerializer
  end
end

users_controller.rb

class UsersController < ApplicationController
  def show
    render json: @user, serializer: UserSerializer
  end
end