@Entity
@Inheritance(strategy = InheritanceType.SINGLE_TABLE)
@DiscriminatorFormula("case when vip_number is not null then 'vip' else 'normal' end")
@DiscriminatorValue("normal")
@IdClass(CustomerPK.class)
public class CustomerWithIdClass implements Serializable {

    private String firstName;
    private String lastName;

    @Id
    private Long versionId;
    @Id
    private Long unitId;

    protected CustomerWithIdClass() {
    }

   // getter and setters ......
}

@NoArgsConstructor
@EqualsAndHashCode
@Embeddable
public class CustomerPK implements Serializable {

    private Long unitId;

    private Long versionId;

    public void setUnitId(Long unitId) {
        this.unitId = unitId;
    }

    public void setVersionId(Long versionId) {
        this.versionId = versionId;
    }
}

@Entity
@DiscriminatorValue("vip")
public class VipCustomerWithIdClass extends CustomerWithIdClass {
    private String vipNumber;

    public VipCustomerWithIdClass() {
    }

    public VipCustomerWithIdClass(String firstName, String lastName, String vipNumber) {
        super(firstName, lastName);
        this.vipNumber = vipNumber;
    }

    public String getVipNumber() {
        return vipNumber;
    }

    public void setVipNumber(String vipNumber) {
        this.vipNumber = vipNumber;
    }
}

        CustomerWithIdClass customer = new CustomerWithIdClass("a", "b");
        customer.setVersionId(123L);
        customer.setUnitId(456L);

        repository.save(customer);//save object of base class, ok

        customer.setFirstName("a2");
        repository.save(customer);//modify object of base class and save again, ok

        VipCustomerWithIdClass vipCustomer = new VipCustomerWithIdClass("a", "b", "888");
        vipCustomer.setVersionId(987L);
        vipCustomer.setUnitId(654L);

        repository.save(vipCustomer);//save object of subclass, ok

        vipCustomer.setVipNumber("999");
        repository.save(vipCustomer);//modify object of subclass and save again, NOT OK
        // ↑ THIS FAILS BECAUSE OF PRIMARY KEY CONFLICT. INSERT STATEMENT WAS USED INSTEAD OF UPDATE, WHY?
        // this failure only happens when:
        // 1. base class uses IdClass for composite primary key
        // 2. saving an instance of the subclass for the second time after modification