在JPA中持久化多对一映射

Question

我正在为我的应用程序使用hibernate JPA。我有两个实体订单和发票。订单与发票类具有多对一映射。

@Entity
@Table(name="ORDERS",schema ="AMZ")
public class Orders {

    @Id
    @SequenceGenerator(name = "pk_sequence", sequenceName = "ORDERS_SEQ", schema = "AMZ", allocationSize = 1)
    @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "pk_sequence")
    @Column(name = "ORDERS_ID")
    private long id;

    @ManyToOne(fetch = FetchType.EAGER)
    @JoinColumn(name = "INVOICE_ID", referencedColumnName = "INVOICE_ID")
    private Invoice invoice;

    //getters and setters 


@Entity
@Table(name="INVOICE",schema ="AMZ")
public class Invoice {

    @Id
    @SequenceGenerator(name = "pk_sequence", sequenceName = "INVOICE_SEQ", schema = "AMZ", allocationSize = 1)
    @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "pk_sequence")
    @Column(name = "INVOICE_ID")
    private long id;

    @Column(name = "INVOICE_NUMBER")
    private String invoiceNumber;


@OneToMany(fetch = FetchType.LAZY, mappedBy = "invoice")
private Set<Orders> Orders= new HashSet<Orders>(0);
    //getters & setters

当我多次坚持订单时，我会在发票表中提供发票，有时候它会为空。

我收到以下错误

 org.hibernate.TransientPropertyValueException: object references an unsaved transient instance -
 save the transient instance before flushing : com.domain.Orders.invoice -> 
 com.domain.Invoice; nested exception is java.lang.IllegalStateException: 
 org.hibernate.TransientPropertyValueException: object references an unsaved transient instance -
 save the transient instance before flushing : 
 com.domain.Orders.invoice -> com.domain.Invoice

如果存在多对一映射，如何保持空值？