Gem for taggable user in comment (facebook-like)

Hi guys,

I currently have an app which has a social network, and I would like
users to be able to tag another user when commenting a post, so I can
notify him.

Is there any gem that can help me with that tagging functionality? Like
begin typing a friend name, the app shows the options, and when the user
selects the option it becomes a link.

Regards,
Rafael