rails通过关联，包括相同的表

Question

class Film < ActiveRecord::Base
  has_many :film_casts
  has_many :derectors, :through => :film_casts, :source => :cast, :conditions => ['film_casts.type_id = 0']
  has_many :performers, :through => :film_casts, :source => :cast, :conditions => ['film_casts.type_id = 1']
end

class Cast < ActiveRecord::Base
end  

class FilmCast < ActiveRecord::Base
  # type_id  0 => derector, 1 => performer

  belongs_to :film
  belongs_to :cast
end

irb(main):001:0> films = Film.includes(:performers, :derectors).limit(5)

  Film Load (0.3ms)  SELECT `films`.* FROM `films` LIMIT 2
  FilmCast Load (0.3ms)  SELECT `film_casts`.* FROM `film_casts` WHERE `film_casts`.`film_id` IN (1, 2) AND (film_casts.type_id = 1)
  Cast Load (0.4ms)  SELECT `casts`.* FROM `casts` WHERE `casts`.`id` IN (2, 3, 4, 5, 6, 7, 9, 10, 11, 12)

问题是，为什么它只包括:performers？我怎样才能在同一张表中包含两者？