This illustration shows the object model used in the unit of work examples provided in this chapter. There are three classes: Pet, PetOwner, and VetVisit. The Pet class provides an integer id, String name, String type, and references PetOwner and VetVisit. The PetOwner class provides an integer id, String name, and String phoneNumber. The VetVisit class provides an integer id, String symptoms, and String notes and references Pet.