Rails 3 has_many :通过表单

无法弄清楚为什么这不起作用.第一次使用 :has_many => :through

Can't figure out why this is not working. First time using :has_many => :through

不断获取未初始化的常量 User::Employmentship

class User < ActiveRecord::Base
  has_many :employmentships
  has_many :companies, :through => :employmentships
  accepts_nested_attributes_for :employmentships, :allow_destroy => true, :reject_if => proc { |obj| obj.blank? }
  attr_accessible :email, :password, :password_confirmation, :firstname, :lastname, :username,  :role, :company_ids
end

class Company < ActiveRecord::Base
  has_many :employmentships
  has_many :users, :through => :employmentships
end


/views/users/_form.html.erb
<p>
   <%= for company in Company.all do %>
     <%= check_box_tag "user[company_ids][]", company.id, @user.companies.include?(company) %>
     <%= company.name%>
   <% end %>
</p>

编辑 - 如果我将 @user.companies.include?(company) 更改为 false，我会收到表单，但没有任何更新.

EDIT - If I change @user.companies.include?(company) to false i get the form, but nothing updates.

编辑 2 -

class Employmentship < ActiveRecord::Base
  belongs_to :company
  belongs_to :user
  attr_accessor :company_id, :user_id
end