title: Attribute level authorization using cancancan in rails
body: <p>I am using cancan for authorization.
  I have different admin roles and I want to show them specific columns to them according to their role(admin, mega_admin). Basically I want to do something like this.</p>
  
  <pre><code>index do
   selectable_column
   column :first_column if can? :read_first_column #visible to mega_admin only
   column :second_column if can? :read_second_column
   actions
  end
  </code></pre>
  
  <p>How do i do this since cancancan doesn't provide attribute level authorization?</p>