jpa OneToMany的简单问题

订单项package cn.itcast.bean.OneToMany;import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.ManyToOne;@Entity
public class OrderItem {
private int id;
private String productName;
private Double sellPrice;
private Order order; @Id @GeneratedValue
public int getId() {
return id;
} public void setId(int id) {
this.id = id;
} @Column(length=40,nullable=false)
public String getProductName() {
return productName;
} public void setProductName(String productName) {
this.productName = productName;
} @Column(nullable=false)
public Double getSellPrice() {
return sellPrice;
} public void setSellPrice(Double sellPrice) {
this.sellPrice = sellPrice;
} @ManyToOne(cascade={CascadeType.MERGE,CascadeType.REFRESH},optional=false)
@JoinColumn(name="order_id")
public Order getOrder() {
return order;
} public void setOrder(Order order) {
this.order = order;
}


}订单package cn.itcast.bean.OneToMany;import java.util.HashSet;
import java.util.Set;import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.Id;
import javax.persistence.OneToMany;
import javax.persistence.Table;import sun.jdbc.odbc.OdbcDef;@Entity
@Table(name="orders")
public class Order {
private String orderId;
private Double amount;
private Set<OrderItem> items = new HashSet<OrderItem>(); @Id @Column(length = 12)
public String getOrderId() {
return orderId;
} public void setOrderId(String orderId) {
this.orderId = orderId;
} @Column(nullable = false)
public Double getAmount() {
return amount;
} public void setAmount(Double amount) {
this.amount = amount;
} @OneToMany(cascade = { CascadeType.REFRESH, CascadeType.PERSIST,
CascadeType.MERGE, CascadeType.REMOVE },mappedBy="order")
public Set<OrderItem> getItems() {
return items;
} public void setItems(Set<OrderItem> items) {
this.items = items;
}

public void add(OrderItem orderItem) {
this.items.add(orderItem);
}
}测试
@Test
public void save() {
EntityManagerFactory factory = Persistence.createEntityManagerFactory("itcast");
EntityManager em = factory.createEntityManager();
em.getTransaction().begin();

Order order = new Order();
order.setAmount(567.0);
order.setOrderId("888");

OrderItem oi1 = new OrderItem();
oi1.setProductName("脚踏车");
oi1.setSellPrice(123.0);

OrderItem oi2 = new OrderItem();
oi2.setProductName("宝马");
oi2.setSellPrice(444.0);

order.add(oi1);
order.add(oi2);

em.persist(order);

em.getTransaction().commit();
em.close();
factory.close();
}
报错
javax.persistence.PersistenceException: org.hibernate.PropertyValueException: not-null property references a null or transient value: cn.itcast.bean.OneToMany.OrderItem.order
at org.hibernate.ejb.AbstractEntityManagerImpl.throwPersistenceException(AbstractEntityManagerImpl.java:614)
at org.hibernate.ejb.AbstractEntityManagerImpl.persist(AbstractEntityManagerImpl.java:226)
at junit.test.OneToManyTest.save(OneToManyTest.java:44)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at org.junit.internal.runners.TestMethodRunner.executeMethodBody(TestMethodRunner.java:99)
at org.junit.internal.runners.TestMethodRunner.runUnprotected(TestMethodRunner.java:81)
at org.junit.internal.runners.BeforeAndAfterRunner.runProtected(BeforeAndAfterRunner.java:34)
at org.junit.internal.runners.TestMethodRunner.runMethod(TestMethodRunner.java:75)
at org.junit.internal.runners.TestMethodRunner.run(TestMethodRunner.java:45)
at org.junit.internal.runners.TestClassMethodsRunner.invokeTestMethod(TestClassMethodsRunner.java:66)
at org.junit.internal.runners.TestClassMethodsRunner.run(TestClassMethodsRunner.java:35)
at org.junit.internal.runners.TestClassRunner$1.runUnprotected(TestClassRunner.java:42)
at org.junit.internal.runners.BeforeAndAfterRunner.runProtected(BeforeAndAfterRunner.java:34)
at org.junit.internal.runners.TestClassRunner.run(TestClassRunner.java:52)
at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:38)
at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:460)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:673)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:386)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:196)
Caused by: org.hibernate.PropertyValueException: not-null property references a null or transient value: cn.itcast.bean.OneToMany.OrderItem.order
at org.hibernate.engine.Nullability.checkNullability(Nullability.java:95)
at org.hibernate.event.def.AbstractSaveEventListener.performSaveOrReplicate(AbstractSaveEventListener.java:313)
at org.hibernate.event.def.AbstractSaveEventListener.performSave(AbstractSaveEventListener.java:204)
at org.hibernate.event.def.AbstractSaveEventListener.saveWithGeneratedId(AbstractSaveEventListener.java:130)
at org.hibernate.ejb.event.EJB3PersistEventListener.saveWithGeneratedId(EJB3PersistEventListener.java:49)
at org.hibernate.event.def.DefaultPersistEventListener.entityIsTransient(DefaultPersistEventListener.java:154)
at org.hibernate.event.def.DefaultPersistEventListener.onPersist(DefaultPersistEventListener.java:110)
at org.hibernate.impl.SessionImpl.firePersist(SessionImpl.java:636)
at org.hibernate.impl.SessionImpl.persist(SessionImpl.java:628)
at org.hibernate.engine.EJB3CascadingAction$1.cascade(EJB3CascadingAction.java:28)
at org.hibernate.engine.Cascade.cascadeToOne(Cascade.java:291)
at org.hibernate.engine.Cascade.cascadeAssociation(Cascade.java:239)
at org.hibernate.engine.Cascade.cascadeProperty(Cascade.java:192)
at org.hibernate.engine.Cascade.cascadeCollectionElements(Cascade.java:319)
at org.hibernate.engine.Cascade.cascadeCollection(Cascade.java:265)
at org.hibernate.engine.Cascade.cascadeAssociation(Cascade.java:242)
at org.hibernate.engine.Cascade.cascadeProperty(Cascade.java:192)
at org.hibernate.engine.Cascade.cascade(Cascade.java:153)
at org.hibernate.event.def.AbstractSaveEventListener.cascadeAfterSave(AbstractSaveEventListener.java:479)
at org.hibernate.event.def.AbstractSaveEventListener.performSaveOrReplicate(AbstractSaveEventListener.java:357)
at org.hibernate.event.def.AbstractSaveEventListener.performSave(AbstractSaveEventListener.java:204)
at org.hibernate.event.def.AbstractSaveEventListener.saveWithGeneratedId(AbstractSaveEventListener.java:144)
at org.hibernate.ejb.event.EJB3PersistEventListener.saveWithGeneratedId(EJB3PersistEventListener.java:49)
at org.hibernate.event.def.DefaultPersistEventListener.entityIsTransient(DefaultPersistEventListener.java:154)
at org.hibernate.event.def.DefaultPersistEventListener.onPersist(DefaultPersistEventListener.java:110)
at org.hibernate.event.def.DefaultPersistEventListener.onPersist(DefaultPersistEventListener.java:61)
at org.hibernate.impl.SessionImpl.firePersist(SessionImpl.java:645)
at org.hibernate.impl.SessionImpl.persist(SessionImpl.java:619)
at org.hibernate.impl.SessionImpl.persist(SessionImpl.java:623)
at org.hibernate.ejb.AbstractEntityManagerImpl.persist(AbstractEntityManagerImpl.java:220)
... 21 more
在测试方法中把oi1和oi2加上oi1.setOrder(order);oi2.setOrder(order);就可以
但是jpa不是可以实现关联的吗？