One to many map in Hibernate - error

can anyone tell me where is the error in this example

@Entity@Table(name = "ITEM")public class Item  implements Serializable{@Id@GeneratedValue@Column(name = "ID")private Long id;@Column(name = "NAME")private String name;@OneToMany(cascade = CascadeType.ALL)@JoinColumn(name = "ID_ITEM",referencedColumnName="ID")private List<ItemDetail> itemDetails;

second class

 @Entity    @Table(name = "ITEM_DETAIL")    public class ItemDetail  implements Serializable    {    @Id    @GeneratedValue    @Column(name = "ID")    private Long id;    @Column(name = "NAME")    private String name;    @Column(name = "ID_ITEM")    private Long itemId;

and the db

COMMIT;CREATE TABLE item(id serial PRIMARY KEY,name VARCHAR(16));CREATE TABLE item_detail(ID serial PRIMARY KEY,NAME VARCHAR(16),ID_ITEM serial REFERENCES item (id));COMMIT;

The error i got is

Hibernate: select nextval ('hibernate_sequence')Hibernate: select nextval ('hibernate_sequence')Hibernate: select nextval ('hibernate_sequence')Hibernate: insert into ITEM (NAME, ID) values (?, ?)Hibernate: insert into ITEM_DETAIL (ITEM_ID, NAME, ID) values (?, ?, ?)Hibernate: insert into ITEM_DETAIL (ITEM_ID, NAME, ID) values (?, ?, ?)Exception in thread "main" org.hibernate.exception.SQLGrammarException: Could not execute JDBC batch update    at org.hibernate.exception.SQLStateConverter.convert(SQLStateConverter.java:90)    at org.hibernate.exception.JDBCExceptionHelper.convert(JDBCExceptionHelper.java:66)    at org.hibernate.jdbc.AbstractBatcher.executeBatch(AbstractBatcher.java:275)    at org.hibernate.engine.ActionQueue.executeActions(ActionQueue.java:266)    at org.hibernate.engine.ActionQueue.executeActions(ActionQueue.java:167)    at org.hibernate.event.def.AbstractFlushingEventListener.performExecutions(AbstractFlushingEventListener.java:321)    at org.hibernate.event.def.DefaultFlushEventListener.onFlush(DefaultFlushEventListener.java:50)    at org.hibernate.impl.SessionImpl.flush(SessionImpl.java:1027)    at org.hibernate.impl.SessionImpl.managedFlush(SessionImpl.java:365)    at org.hibernate.transaction.JDBCTransaction.commit(JDBCTransaction.java:137)    at com.mkyong.common.App.main(App.java:51)    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 com.intellij.rt.execution.application.AppMain.main(AppMain.java:115)Caused by: java.sql.BatchUpdateException: Batch entry 0 insert into ITEM_DETAIL (ITEM_ID, NAME, ID) values (NULL, id1, 161) was aborted.  Call getNextException to see the cause.    at org.postgresql.jdbc2.AbstractJdbc2Statement$BatchResultHandler.handleError(AbstractJdbc2Statement.java:2530)    at org.postgresql.core.v3.QueryExecutorImpl.processResults(QueryExecutorImpl.java:1317)    at org.postgresql.core.v3.QueryExecutorImpl.execute(QueryExecutorImpl.java:350)    at org.postgresql.jdbc2.AbstractJdbc2Statement.executeBatch(AbstractJdbc2Statement.java:2592)    at org.hibernate.jdbc.BatchingBatcher.doExecuteBatch(BatchingBatcher.java:70)    at org.hibernate.jdbc.AbstractBatcher.executeBatch(AbstractBatcher.java:268)    ... 13 moreProcess finished with exit code 1

It's obviously that item_id is null but why??

ThanksRegards