Rails test fail

Question

The code works fine but i can't understand why the test fails.

This is my test:

ab

and this is the new action in Orders controller:

 test "requires item in cart" do
    get :new
    assert_redirected_to store_path
    assert_equal flash[:notice], 'Your cart is empty'
  end

and i get an error:

def new
    if @cart.line_items.empty?
      redirect_to store_url, notice: "Your cart is empty"
      return
    end

    @order = Order.new
  end

models associations:

"test_requires_item_in_cart", OrdersControllerTest, 2015-07-28 20:21:48 +0300]
 test_requires_item_in_cart#OrdersControllerTest (1438104108.20s)
NoMethodError:         NoMethodError: undefined method `line_items' for 3:Fixnum

and

class LineItem < ActiveRecord::Base
  belongs_to :product
  belongs_to :cart

I think the problem is that the test cannot recognize the association between @cart and line_items in controller.

Any help?