problem with through.

Has anyone ever had problems with :through relationship?
I have set it properly, according to the book, but getting response
from the browser

"Could not find the association :line_items in model Product"

and the relationship set in Product model is:
has_many :orders, :through => :line_items

Order model looks like:
has_many :line_items

and LineItem
belongs_to :product
belongs_to :order

I want to find orders which contain product with an id = 3
I am doing it through a method

def who_bought
@product = Product.find(params[:id])
@orders = @product.orders
end

and then I have an rxml template to iterate over the orders and print
them onto the screen.
Help please!

Wow, thanks,
that helped. Im totally new in RoR and as much as it's all features
seems to be helpful, they're also confusing sometimes.

Cheers