How do I create a Rails action for an Ajax request? Rails 3.2.1

I have a list of items in my shopping page, and each time an "add to
cart" button is clicked, an AJAX request is fired off with only a
product_id.

"Add this item to the cart" so to speak.

What does my Action have to look like in Rails for this scenario?

I'm specifically asking about the controller side, and what the action
would look like. Just assume it receives a numerical ID.

def action_name
    respond_to do |format|
      format.js do
  ...
      end
    end
  end

Jesus it's that simple? I'll come back with my results!

Of course you have also got to do whatever it is that you actually
want to achieve - finding the appropriate record or whatever.

Coin