@PostMapping(value = "/owners/{ownerId}/pets/{petId}/visits/new")
    public String processNewVisitForm(@Valid Visit visit, BindingResult result) {
        if (result.hasErrors()) {
            return "pets/createOrUpdateVisitForm";
        }

        this.clinicService.saveVisit(visit);
        return "redirect:/owners/{ownerId}";
    }
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