@Override
    public void save(Owner owner) {
        if (owner.getId() == null) {
            this.em.persist(owner);
        } else {
            this.em.merge(owner);
        }

    }
Important: in the current version of this method, we load Owners with all their Pets and Visits while
we do not need Visits at all and we only need one property from the Pet objects (the 'name' property).
There are some ways to improve it such as:
- creating a Ligtweight class (example here: https://community.jboss.org/wiki/LightweightClass)
- Turning on lazy-loading and using {@link OpenSessionInViewFilter}
src/main/java/org/springframework/samples/petclinic/repository/jpa/JpaOwnerRepositoryImpl.java