How to tell if something has been read in Rails for notification

Question

I'd like to implement a simple notification system on my site where an unseen/unread item is displayed to the user. Similar to the one used across Stack Exchange for the user's inbox where unread comments on questions, etc are displayed.

I came across this question that provides an overview of how I'd do this. What I'm confused about is how to figure out if something has been read. I could add a read_at column but how do I actually fill it? If anyone could help me with some basic guidance I'd appreciate it!

UPDATE #1: What if I add a conditional to my Item#show action where I check the user_id (ID of the user creating the item) against current_user.id. Something like the below:

unless @item.user_id == current_user.id
  @item.read_at = Time.now
end

UPDATE #2: Using the code below, I'm attempting to update the message's read_at if its recipient_id matches the current_user ID. However it's not working.

def show
  @message = Message.find(params[:id])
  if @message.recipient_id == current_user.id
    @message.read_at == Time.now
    @message.save
  end
  respond_to do |format|
    format.html # show.html.erb
    format.xml  { render :xml => @message }
  end
end

FINAL UPDATE: Thanks to @prasvin, here's my solution. I added a read_at column to the object. The object also has an existing recipient_id column. So in my Controller's show action, I put the following:

def show
  @message = Message.find(params[:id])
  if @message.recipient_id == current_user.id
    @message.update_attributes(:read_at => Time.now)
  end
  respond_to do |format|
    format.html # show.html.erb
    format.xml  { render :xml => @message }
  end
end

Then in my helper.rb file:

def new_messages
  Message.where(:recipient_id => current_user.id, :read_at => nil).count
end

And in my layout:

<% if new_messages > 0 %><span class="notification"><%= new_messages %></span><% end %>

Answer 1

How about filling in read_at column in show action, i.e. we have the object in the show action,and then update its read_at attribute before redering the page