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   acts_as_ferret(     :fields => {        :ferret_tags => {boost, whatever},        other fields     }   )

  def ferret_tags     tags.join " "   end