问我不明白为什么我得到了未定义的方法form_for

EN

undefined method `sector_id' for #<Portfolio:0x007fe17c2e3848>

class Portfolio < ActiveRecord::Base
  belongs_to :sector
  attr_accessible :overview, :title, :sector_id
end



class Sector < ActiveRecord::Base
  has_many :portfolios
  attr_accessible :name
end

resources :portfolios do
  resources :sectors
end

<%= f.label :sector_id, "Choose Sector", :class => 'title_label' %><br>
<%= f.collection_select(:sector_id, Sector.all, :id, :name, :prompt => "Please Select a Sector") %>