model :

cart.rb  
   class Cart < ActiveRecord::Base
   attr_accessible :title, :body

    has_many :line_items, dependent: :destroy
   end

product.rb
 class Product < ActiveRecord::Base

  default_scope :order => 'title'


  has_many :line_items 
  before_destroy :ensure_not_referenced_by_any_line_item
  attr_accessible :description, :image_url, :price, :title


  validates :title, :description, :image_url,:presence => true
  validates :price,:numericality => {:greater_than_or_equal_to => 0.01}
  validates :title,:uniqueness => true
  validates :image_url,:format => {
    :with  =>%r{\.(gif|jpg|png)$}i,
     :message =>'must be a url for GIF ,JPG OR PNG image_url'}

private 

    def ensure_not_referenced_by_any_line_item
       if line_items.empty?
         return true
       else 
        errors.add(:base ,'line items present')
        return false          
        end    
    end


line_item.rb
 class LineItem < ActiveRecord::Base
  attr_accessible  :cart_id, :product_id,:quantity


  belongs_to :Product
  belongs_to :cart
end



controller

 line_item_controller.rb 中的creat方法
def create
   @cart = current_cart
   product=Product.find(params[:product_id])
    @line_item = @cart.line_items.build                     (这儿是第45行)
    @line_item.product = product

   respond_to do |format|
    if @line_item.save
      format.html  { redirect_to(@line_item.cart,
        :notice =>  'line item was successfully created.')}
      format.xml {render :xml => @line_item,
        :status => :created ,:location => @line_item}
    else
      format.html {render :action => "new"}
      format.xml {render :xml => @line_item.errors,
        :status => :unprocessable_entity}
      end
    end
  end

运行是出现 ActiveRecord::UnknownAttributeError in LineItemsController#create

unknown attribute: cart_id app/controllers/line_items_controller.rb:45:in `create'

我 google 了下 还是每搞懂哪儿错了。