问为什么Hibernate包含一个带有插入的标识字段？

EN

@Entity
@Table(name = "CSU_AUDIT")
@NamedQuery(name = "findAllEventsForObjectId", query = "from AuditEvent e where e.objectId = :objectId order by e.auditTimestamp desc")
public class AuditEvent implements Serializable {
    /**
     * 
     */
    private static final long serialVersionUID = 577051869176721340L;

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private int auditId;

    @Column(name = "INSERT_TS")
    private Date auditTimestamp;

    @Column(name = "OBJECT_ID")
    private String objectId;

    @Column(name = "ACTION_CODE")
    private String actionCode;

    @Column(name = "USER_ID")
    private String userId;

    @Column(name = "FIELD_NAME")
    private String fieldName;

    @Column(name = "OLD_VALUE")
    private String oldValue;

    @Column(name = "NEW_VALUE")
    private String newValue;

public void insertEvent(AuditEvent event) {
        final Session session = sessionFactory.getCurrentSession();
        session.beginTransaction();
        log.info("Saving audit event...");
        log.debug(event);
        session.save(event);
        session.getTransaction().commit();
    }

2012-07-12 09:01:41,049 DEBUG org.hibernate.event.def.AbstractSaveEventListener - executing identity-insert immediately
2012-07-12 09:01:41,053 DEBUG org.hibernate.jdbc.AbstractBatcher - about to open PreparedStatement (open PreparedStatements: 0, globally: 0)
2012-07-12 09:01:41,066 DEBUG org.hibernate.SQL - insert into CSU_AUDIT (auditId, ACTION_CODE, INSERT_TS, FIELD_NAME, NEW_VALUE, OBJECT_ID, OLD_VALUE, USER_ID) values (default, ?, ?, ?, ?, ?, ?, ?)
2012-07-12 09:01:41,100 DEBUG org.hibernate.jdbc.AbstractBatcher - about to close PreparedStatement (open PreparedStatements: 1, globally: 1)
2012-07-12 09:01:41,117 DEBUG org.hibernate.util.JDBCExceptionReporter - could not insert: [com.cinfin.ecm.csudocmgr.audit.AuditEvent] [insert into CSU_AUDIT (auditId, ACTION_CODE, INSERT_TS, FIELD_NAME, NEW_VALUE, OBJECT_ID, OLD_VALUE, USER_ID) values (default, ?, ?, ?, ?, ?, ?, ?)]
com.ibm.db2.jcc.c.SqlException: DB2 SQL error: SQLCODE: -206, SQLSTATE: 42703, SQLERRMC: AUDITID