Consider the model in Figure $6-24$ which describes purchases of a product by customers of a small mail

order company selling toys. For simplicity, each purchase is for one or more of a single toy. Each transaction

must have a customer so that he or she can be invoiced and the product delivered. The data will be used

to prepare statistics about the different products sold, values of purchases, and the spending habits of

customers.

Figure $6-24.$ Customers purchase products.

The company changes the way it does business to allow customers to walk in off the street and pay cash.

No customer needs to be associated with a cash purchase. Discuss how effective the following changes to

the data would be$.$

Change the optionality at the customer end of the relationship to $0$ so not all

purchases need a customer.

Leave the optionality as $1$ but include a dummy customer object, with name

CashCustomer.

Create subclasses of Customer: Cash$\_$Customer and Account$\_$Customer.

Create subclasses of Purchase: Cash$\_$Purchase and Account$\_$Purchase.