Updating ActiveRecord in Rails test

I'm writing my first Rails 'Store' app and I'm coming across something weird in one of my tests. I'm trying to test the add_product method on cart.rb:

    class Cart < ActiveRecord::Base
  has_many :line_items, :dependent => :destroy

  def add_product(product)
    current_item = line_items.find_by_product_id(product.id)
    if current_item
      current_item.quantity += 1
    else
      current_item = line_items.build(:product_id => product.id, :price => product.price)      
    end
    current_item
  end

  def total_price 
    line_items.to_a.sum { |item| item.total_price }
  end

end

I'm testing that adding the same product to a cart twice increases the quantity rather than adding a new line (line_item). Here's the test:

test "line item quantity is increased when duplicate product is added" do
    cart = carts(:one)
    ruby_book = products(:ruby)

    cart.add_product(ruby_book)
    cart.save    
    assert_equal 1, cart.line_items(ruby_book).to_a.count

    c开发者_高级运维art.add_product(ruby_book)
    cart.save
    assert_equal 1, cart.line_items(ruby_book).to_a.count
    assert_equal 2, cart.line_items(ruby_book).first.quantity # fail!
  end

I get a fail on the last assert. The quantity is 1, where I expect 2. I've checked the test.log file and no update is ever run against my sqlite3 instance... I can add the log or the Fixtures files if necessary, but I'm sure this is such a newbie question that it won't be required!

Thanks in advance,

Stu

---

You are modifying the line item's quantity attribute, but not saving the change. Calling save on the parent won't save the children's attributes. You can call save on current_item in the add_product method after the increment line.

if current_item
  current_item.quantity += 1
  current_item.save
else