title: Can Can selectively cache abilities
body: <p>I know about caching the whole <code>current_ability</code>.</p>
  
  <p>Something like</p>
  
  <pre><code>def current_ability
    cached_ability = Rails.cache.read("#{current_user.cache_key}::ability")
    if cached_ability.present?
      ability = Marshal.load( cached_ability )
    else
      ability = super
      Rails.cache.write("#{current_user.cache_key}::ability", Marshal.dump( ability ))
    end
    @current_ability = ability
  end
  </code></pre>
  
  <p>But is there is a way to cache a single ability ?
  for example I do this :</p>
  
  <pre><code>Rails.cache.fetch("#{user.cache_key}::comments::ability") do
    can :manage, Comment, :article_id =&gt; articles_ids
  end
  </code></pre>