code: def mark_as_unread(user)
        if previous_post.nil?
          read_state = postable.user_read_states.find_by(user_id: user.id)
          read_state.destroy if read_state
        else
          postable.user_read_states.touch!(user.id, previous_post, overwrite_newer: true)
        end
      end
docs: Marks all the posts from the given one as unread for the given user
   @param [Thredded.user_class] user