@Entity
@Table(name = "calendar_users")
public class User extends BaseEntity {

    @Column(nullable = false)
    private String name;

    @OneToMany(mappedBy = "user")
    private List<Attendee> attendeeEvents;

    @OneToMany(mappedBy = "organizer")
    private List<Event> organizedEvents;
}

@Entity
@Table(name = "events")
public class Event extends BaseEntity {

    @Column(nullable = false)
    private String name;

    @ManyToOne(optional = false)
    @JoinColumn(name = "user_id")
    private User organizer;

    @OneToMany(mappedBy = "event")
    private List<Attendee> attendees;
}

@Entity
@Table(name = "attendees")
public class Attendee extends BaseEntity {

    private Date responseTime;

    @ManyToOne(optional = false)
    @JoinColumn(name = "response_status_id")
    private ResponseStatus responseStatus;

    @ManyToOne(optional = false)
    @JoinColumn(name = "user_id")
    private User user;

    @ManyToOne(optional = false)
    @JoinColumn(name = "event_id")
    private Event event;