The famous myCafé start-up has hired us to start developing their core business. The myCafé business idea is very simple: allow customers to preorder coffee from a mobile web page so that, when they arrive at the coffee shop, it will already be waiting.

This involves several subsystems, such as payment, orders, shop, inventory, and so on. We have been charged with the task of developing the orders subsystem.

The order subsystem is clearly not very complicated. The user goes to the ordering page, selects the products they want, and then places the order. Users can add drinks to their order, remove them, and change the quantity desired for each. The order subsystem can also place the order, triggering the payment process; after that, it will communicate the order to the shop.

We will focus only on the basic functionality mentioned earlier. We will not deal with other more advanced functionality such as having several orders or being able to select between several shops. This...