HABTM联接表的机械化批量分配错误

问题是我得到了这个错误:

ActiveModel::MassAssignmentSecurity::Error: Can't mass-assign protected attributes: amenity_id

当我运行此代码时:

task import_amenities: :environment do

  agent = Mechanize.new

  Kindergarten.find_all_by_public(false).each do |k| 
    p = agent.get(k.uri)
    amenities = p.search("td td tr:nth-child(11) td:nth-child(2)").text.split(/(;|,) */)
    amenities.each do |a|
      am = Amenity.find_or_create_by_name!("#{a}")
      k.update_attributes(amenity_id: am.id)
    end
  end
end

幼儿园和便利设施通过HABTM关系联系在一起,定义如下:

幼儿园.rb

class Kindergarten < ActiveRecord::Base
  attr_accessible :location, :name, :public, :uri, :address, :contact, 
                  :phone, :url, :email, :description, 
                  :password, :password_confirmation, :amenity_ids
  has_and_belongs_to_many :amenities
end

便利设施.rb

class Amenity < ActiveRecord::Base
  attr_accessible :name, :kindergarten_ids
  has_and_belongs_to_many :kindergartens
end

下面是联接表的迁移:

class CreateKindergartensAmenitiesJoinTable < ActiveRecord::Migration
  def up

    create_table :kindergartens_amenities, :id => false do |t|
      t.integer :kindergarten_id
      t.integer :amenity_id
    end
  end
end

该错误是由rake任务中的这一行引起的:

k.update_attributes(amenity_id: am.id)

在我完成大规模任务之前,控制台上的一切似乎都很好。我觉得我真的搞砸了,因为我以前从未使用过HABTM。

有什么想法吗?