from the Agile Web Development with Rails, Third Edition. (fourth printing), in Chapter 8.3 iteration C2: Creating a Smarter Cart (page 104), when creating the initial CartItem model class, the price function is defined thusly :
def price
@product.price = @quantity
end
Unfortunately that sets the price to the number of items set in the cart (if you add the item 3 times the price is 3, not $99.88). It should be :
def price
@product.price * @quantity
end
or you could do
def price
@product.price
end
If you do it the second way, the total_price needs to be adjusted (8.5 Iteration C4:finishing the Cart, page 114) be modifying the total_price function from:
def total_price
@items.sum { |item| item.price }
end
to the correct :
def total_price
@items.sum { |item| item.price * item.quantity }
end
see http://www.pragprog.com/titles/rails3/errata for more corrections of this otherwise great guide.