gistfile1.txt

# MODEL

class Theater
  named_scope :with_recent_comments, 
    :select => "*, recent_comments.created_at AS last_comment_at",
    :joins  => "INNER JOIN (SELECT id, entity_id, created_at FROM comments WHERE entity_type = 'Theater' ORDER by id DESC LIMIT 100) 
                AS recent_comments ON recent_comments.entity_id = theaters.id",
    :order  => "recent_comments.created_at DESC"
end

# CONTROLLER

Theater.with_recent_comments