свойство not-null ссылается на нулевое или переходное значение

проблемы с сохранением родительского/дочернего объекта с помощью hibernate. Любая идея будет высоко оценена.

org.hibernate.PropertyValueException: not-null property references a null or transient value: example.forms.InvoiceItem.invoice

    at org.hibernate.engine.Nullability.checkNullability(Nullability.java:100)

        .... (truncated)

спящий режим отображения:

<hibernate-mapping package="example.forms">

    <class name="Invoice" table="Invoices">

        <id name="id" type="long">

            <generator class="native" />

        </id>

        <property name="invDate" type="timestamp" />

        <property name="customerId" type="int" />

        <set cascade="all" inverse="true" lazy="true" name="items" order-by="id">

            <key column="invoiceId" />

            <one-to-many class="InvoiceItem" />

        </set>

    </class>

    <class name="InvoiceItem" table="InvoiceItems">

        <id column="id" name="itemId" type="long">

            <generator class="native" />

        </id>

        <property name="productId" type="long" />

        <property name="packname" type="string" />

        <property name="quantity" type="int" />

        <property name="price" type="double" />

        <many-to-one class="example.forms.Invoice" column="invoiceId" name="invoice" not-null="true" />

    </class>

</hibernate-mapping>

InvoiceManager.java

class InvoiceManager {



    public Long save(Invoice theInvoice) throws RemoteException {

        Session session = HbmUtils.getSessionFactory().getCurrentSession();

        Transaction tx = null;

        Long id = null;

        try {

            tx = session.beginTransaction();

            session.persist(theInvoice);

            tx.commit();

            id = theInvoice.getId();

        } catch (RuntimeException e) {

            if (tx != null)

                tx.rollback();

            e.printStackTrace();

            throw new RemoteException("Invoice could not be saved");

        } finally {

            if (session.isOpen())

                session.close();

        }

        return id;

    }

}

счет-фактура.java

public class Invoice implements java.io.Serializable {

    private Long id;

    private Date invDate;

    private int customerId;

    private Set<InvoiceItem> items;



    public Long getId() {

        return id;

    }

    public Date getInvDate() {

        return invDate;

    }

    public int getCustomerId() {

        return customerId;

    }

    public Set<InvoiceItem> getItems() {

        return items;

    }

    void setId(Long id) {

        this.id = id;

    }

    void setInvDate(Date invDate) {

        this.invDate = invDate;

    }

    void setCustomerId(int customerId) {

        this.customerId = customerId;

    }

    void setItems(Set<InvoiceItem> items) {

        this.items = items;

    }

}

InvoiceItem.java

public class InvoiceItem implements java.io.Serializable {

    private Long itemId;

    private long productId;

    private String packname;

    private int quantity;

    private double price;

    private Invoice invoice;



    public Long getItemId() {

        return itemId;

    }

    public long getProductId() {

        return productId;

    }

    public String getPackname() {

        return packname;

    }

    public int getQuantity() {

        return quantity;

    }

    public double getPrice() {

        return price;

    }

    public Invoice getInvoice() {

        return invoice;

    }

    void setItemId(Long itemId) {

        this.itemId = itemId;

    }

    void setProductId(long productId) {

        this.productId = productId;

    }

    void setPackname(String packname) {

        this.packname = packname;

    }

    void setQuantity(int quantity) {

        this.quantity = quantity;

    }

    void setPrice(double price) {

        this.price = price;

    }

    void setInvoice(Invoice invoice) {

        this.invoice = invoice;

    }

}