Rails - How to make a conditional Radio Button checked?

Given a radio button like so:

<%= radio_button_tag 'permission[role_id]', '2' %>

How can I make the radio buttons checked status condition. To only be checked if @permission.role_id.nil? or if @permission.role_id == 2?

I tried:

<%= radio_button_tag 'permission[role_id]', '2', @permission.role_id.nil? ? {:checked => true} : {:checked => false} %>

Thanks