auth.rb

# controllers/application_controller.rb
class ApplicationController
  # @override
  def current_ability
    @current_ability ||= Ability::Factory.build(user)
  end
end

# ability/factory.rb
class Ability::Factory
  def self.build(user)
    case user.role
      when :user then Ability::User.new(user)
      when :admin then Ability::Admin.new(user)
      when :editor then Ability::Editor.new(user)
    end
  end
end

# ability/editor.rb
class Ability::Editor < Ability
  def initialize(user)
    can :view, Report, last_modifier_id: user.id
  end
end