public UserBean getUser() {
        return user;
    }
Which type of notification does the user want to opt into, and for which types of notification.
<p>
Constraint notes: Each user can only have one {@link NotificationPreferenceEntity} entity for each type of
notification. For example, user 'abc' may only have one 'SMS' preference entity.

@author Marc Savy {@literal <marc@blackparrotlabs.io>}
manager/api/beans/src/main/java/io/apiman/manager/api/beans/notifications/NotificationPreferenceEntity.java