acts_as_ferret search associated tables?

I am currently using acts_as_ferret to search on a Posts table (title
and content fields). These posts also have tags (with has_many :tags,
:through => :questions_tags). I can't figure out how to get
acts_as_ferret to work with that relationship and allow searching of
title, content, And tags at the same time (with the ability to set
boost as well). Can this be done?

class Post < ActiveRecord::Base
    :fields => {
       :ferret_tags => {boost, whatever},
       other fields

  def ferret_tags
    tags.join " "