Ruby on Rails

I am new to Ruby and to Rails (using Ruby 1.9.3 and Rails 3.2.3) so I
picked up the Agile Web Development book and have been working through
it (apologies if this isn't the correct place for this question but the
pragprog forums don't seem to work for me).  I am currently working
through the book and have reached the point where I am adding
functionality to an "Add to Cart" button.  Here is the code that was
provided for the 'create' method in the line_item_controller:

def create

 @cart = current_cart
 product = Product.find(params[:product_id])
 @line_item = @cart.line_items.build(product: product)

...

end

This results in the following error when I click "Add to Cart"

Can't mass-assign protected attributes: product

Can anyone shed some light on what is going on here (my search-fu has
failed me)?  The only way that I have been able to get this to work is
by changing the code (starting at @line_item) to:

 @line_item = @cart.line_items.build
 @line_item.product = product

Is this correct or is it just a band-aid fix that may cause issues going
forward?

Thanks,

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