@Entity
public class Auction implements Serializable {
    private static final long serialVersionUID = 1L;
    
    @Id
    @GeneratedValue(strategy=GenerationType.TABLE)
    private Long id;
}

@Entity
public class Dealer {
    @Id
    private Long id;
    @OneToOne(cascade = CascadeType.ALL)
    @MapsId
    private AppUser user;

@Entity
public class AppUser{    
    @Id
    @GeneratedValue(strategy = GenerationType.TABLE)
    private Long userName;    
}

@Entity
@IdClass(AuctionObserverPK.class)
public class AuctionObserver implements Serializable{
    @Id
    @ManyToOne
    private Auction auction;

    @Id
    @ManyToOne
    private Dealer user;
}

public class AuctionObserverPK implements Serializable{
    private Long auction;
    private Long user;
}