change carts/ viewFile
create a migrate FOR add quantity Field to line_items Table
set defaultValue by manually on migration code
summary the quantity for NEW product added to cart (instead of create new product or line_items row to cart).
rails g migration combine_items_in_cart
summary the quantity for OLD line_items for carts/show.html.erb
add def down at db/migrate/combine_items_in_carts.rb
add   private def invalid_cart    at carts_controller.rb
A final scan of the logs identify no problems.
add EmptyCart function(modify def destory code as well), add unitTest as well.
end Ch10, add total_price AND modify UI of carts page.
use partial template (special way of re-using code about RoR, different from PHP)
redirect to store_url AFTER created a line_item
AJAX for     Add to Cart     button