Rails 3 has_many :through Form

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

Keep getting uninitialized constant 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>

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

EDIT开发者_开发知识库 2 -

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

Where is you employmentship model? has_many_through is for going through another model.