How to set html class attribute in rails controller?

I am beginner in rails. I am using after_action in the notifications controller.
Notifications controller

class NotificationsController < ApplicationController

    layout "posts"

    after_action :read_message, only: [:index]

    def index
        @notifications = Notification.where(:recipient_id => session[:registered_id]).order("created_at DESC")

    end

    def new
        @user = User.find(session[:user_id])
        @notification = @user.notifications.new
    end

    def create
        @user = User.find(session[:user_id])
        @notification = @user.notifications.new notification_params
        if @notification.save
            redirect_to(:controller => "posts", :action => "index")
        else
            render "new"
        end
    end

    private

    def notification_params
        params.require(:notification).permit(:message, :user_id, :recipient_id, :status)
    end

    def read_message
        @notifications = Notification.where(:recipient_id => session[:registered_id]).order("created_at DESC")
        @notifications.read_all
    end

end

notifications#index view

 <% @notifications.each do |notification| %>
   <div class = message_wrapper>
    <p><%= notification.message %></p>
    <p class = "message_details">from <span><%= notification.user.registered_id %></span></p>
  </div>
 <% end %>

Now in the after_action method in the controller, i want to set the class of <div>(currently with class message_wrapper) to message_wrapper_read. How can I do this? I appreciate your answers. Thanks in advance.