Order (DTO) containing all relevant information of a Customer and her Products.
The Customer who has placed this order. Must not be null.
The id of an order. It is a number which is generated by a sequence (see Counter.
The key of the preferred language (either „de“ or „en“) of the Customer. Must not be empty.
List of data Products the Customer want to order. Must not be empty.
The current state of the order. One of OrderState.