@ModelAttribute("visit")
    public Visit loadPetWithVisit(@PathVariable("petId") int petId) {
        Pet pet = this.clinicService.findPetById(petId);
        Visit visit = new Visit();
        pet.addVisit(visit);
        return visit;
    }
Called before each and every @GetMapping or @PostMapping annotated method.
2 goals:
- Make sure we always have fresh data
- Since we do not use the session scope, make sure that Pet object always has an id
(Even though id is not part of the form fields)

@param petId
@return Pet
src/main/java/org/springframework/samples/petclinic/web/VisitController.java