各位大虾好，小弟在用hibernate中出现了一个问题，请帮助解决一下!

这是错误：Hibernate: insert into orderc (amount, estate, orderid, invoice, searchExpress) values (?, ?, ?, ?, ?)
Hibernate: insert into Merchandise (quantity, zheKou, amount, merchandiseid, product) values (?, ?, ?, ?, ?)
15:23:21,625  WARN JDBCExceptionReporter:71 - SQL Error: 1062, SQLState: 23000
15:23:21,625 ERROR JDBCExceptionReporter:72 - Duplicate entry '1' for key 2
org.hibernate.exception.ConstraintViolationException: could not insert: [lewis.Merchandise]
at org.hibernate.exception.SQLStateConverter.convert(SQLStateConverter.java:71)
at org.hibernate.exception.JDBCExceptionHelper.convert(JDBCExceptionHelper.java:43)
at org.hibernate.id.insert.AbstractReturningDelegate.performInsert(AbstractReturningDelegate.java:40)
at org.hibernate.persister.entity.AbstractEntityPersister.insert(AbstractEntityPersister.java:2093)
at org.hibernate.persister.entity.AbstractEntityPersister.insert(AbstractEntityPersister.java:2573)
at org.hibernate.action.EntityIdentityInsertAction.execute(EntityIdentityInsertAction.java:47)
at org.hibernate.engine.ActionQueue.execute(ActionQueue.java:248)
at org.hibernate.event.def.AbstractSaveEventListener.performSaveOrReplicate(AbstractSaveEventListener.java:290)
at org.hibernate.event.def.AbstractSaveEventListener.performSave(AbstractSaveEventListener.java:180)
at org.hibernate.event.def.AbstractSaveEventListener.saveWithGeneratedId(AbstractSaveEventListener.java:108)
at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.saveWithGeneratedOrRequestedId(DefaultSaveOrUpdateEventListener.java:186)
at org.hibernate.event.def.DefaultSaveEventListener.saveWithGeneratedOrRequestedId(DefaultSaveEventListener.java:33)
at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.entityIsTransient(DefaultSaveOrUpdateEventListener.java:175)
at org.hibernate.event.def.DefaultSaveEventListener.performSaveOrUpdate(DefaultSaveEventListener.java:27)
at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.onSaveOrUpdate(DefaultSaveOrUpdateEventListener.java:70)
at org.hibernate.impl.SessionImpl.fireSave(SessionImpl.java:535)
at org.hibernate.impl.SessionImpl.save(SessionImpl.java:523)
at org.hibernate.impl.SessionImpl.save(SessionImpl.java:519)
at manager.OrderMgr.addOrder(OrderMgr.java:35)
at actions.OrderAction.add(OrderAction.java:37)
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.apache.struts.actions.DispatchAction.dispatchMethod(DispatchAction.java:270)
at org.apache.struts.actions.DispatchAction.execute(DispatchAction.java:187)
at org.apache.struts.action.RequestProcessor.processActionPerform(RequestProcessor.java:431)
at org.apache.struts.action.RequestProcessor.process(RequestProcessor.java:236)
at org.apache.struts.action.ActionServlet.process(ActionServlet.java:1196)
at util.MyActionServlet.process(MyActionServlet.java:16)
at org.apache.struts.action.ActionServlet.doGet(ActionServlet.java:414)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:627)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:269)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:188)
at util.HibernateFilter.doFilter(HibernateFilter.java:28)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:215)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:188)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:213)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:172)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:117)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:108)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:174)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:873)
at org.apache.coyote.http11.Http11BaseProtocol$Http11ConnectionHandler.processConnection(Http11BaseProtocol.java:665)
at org.apache.tomcat.util.net.PoolTcpEndpoint.processSocket(PoolTcpEndpoint.java:528)
at org.apache.tomcat.util.net.LeaderFollowerWorkerThread.runIt(LeaderFollowerWorkerThread.java:81)
at org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:689)
at java.lang.Thread.run(Thread.java:619)
Caused by: java.sql.SQLException: Duplicate entry '1' for key 2
at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:2975)
at com.mysql.jdbc.MysqlIO.sendCommand(MysqlIO.java:1600)
at com.mysql.jdbc.ServerPreparedStatement.serverExecute(ServerPreparedStatement.java:1125)
at com.mysql.jdbc.ServerPreparedStatement.executeInternal(ServerPreparedStatement.java:677)
at com.mysql.jdbc.PreparedStatement.executeUpdate(PreparedStatement.java:1357)
at com.mysql.jdbc.PreparedStatement.executeUpdate(PreparedStatement.java:1274)
at com.mysql.jdbc.PreparedStatement.executeUpdate(PreparedStatement.java:1259)
at org.hibernate.id.IdentityGenerator$GetGeneratedKeysDelegate.executeAndExtract(IdentityGenerator.java:73)
at org.hibernate.id.insert.AbstractReturningDelegate.performInsert(AbstractReturningDelegate.java:33)
... 47 more
这是XML<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC 
"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="lewis">
<class name="Merchandise">
<id name="id">
<generator class="native" />
</id>
<property name="quantity" />
<property name="zheKou" />
<property name="amount" />
<many-to-one name="order" column="merchandiseid" />
<many-to-one name="product" unique="true" />
</class>
</hibernate-mapping>
这是实体类:
package lewis;public class Merchandise {
private int id; // 商品自生成ID private Product product; // 产品 private int quantity; // 数量 private float zheKou; // 折扣 private float amount; // 总额

private Order order;

public int getId() {
return id;
} public void setId(int id) {
this.id = id;
} public Product getProduct() {
return product;
} public void setProduct(Product product) {
this.product = product;
} public int getQuantity() {
return quantity;
} public void setQuantity(int quantity) {
this.quantity = quantity;
} public float getAmount() {
return amount;
} public void setAmount(float amount) {
this.amount = amount;
} public float getZheKou() {
return zheKou;
} public void setZheKou(float zheKou) {
this.zheKou = zheKou;
} public Order getOrder() {
return order;
} public void setOrder(Order order) {
this.order = order;
}}这是错误的地方： public void addOrder(List merchandises,Order o){
Session session = null;
try{
session = HibernateFilter.getSession();
session.beginTransaction();
session.save(o);

for(Iterator i = merchandises.iterator();i.hasNext();){
Merchandise m =(Merchandise)i.next();
System.out.println(m.getId());
m.setOrder(o);
session.save(m);
}
System.out.print("----------");
session.getTransaction().commit();
}catch(Exception e){
e.printStackTrace();
session.getTransaction().rollback();
}
}请各位大虾看下，问题在于，第一次插入一点问题都没有，在第二次插入后，就报错了，谢谢！