springmvc 一对多的需求

Question

好的有2个实体类，一对多的关系
public class AuditFactory implements Serializable { /**
 * 
 */
private static final long serialVersionUID = 1L;
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
@Column(name = "audit_factory_id")
private int auditFactoryId;
@NotEmpty
@Column(name = "audit_factory_name")
private String factoryName;
@Column(name = "audit_factory_retailer")
private String factoryRetailer;
@Column(name = "country")
private String country;
@Column(name = "location")
private String location;
@DateTimeFormat(pattern="MM/dd/yyyy HH:ss")
    @NotNull
@Column(name = "audit_next_date")
private Date auditNextDate;
@Min(value=1)
@Column(name = "time_interval")
private int interval;
@OneToMany(mappedBy="auditFactory",fetch=FetchType.LAZY,cascade=CascadeType.ALL)
@LazyCollection(LazyCollectionOption.EXTRA)
private List<AuditReport> auditReports;
public class AuditReport implements Serializable{ /**
 * 
 */
private static final long serialVersionUID = 1L;
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
@Column(name = "audit_report_id")
private int auditReportId;
@ManyToOne(cascade=CascadeType.ALL,fetch=FetchType.LAZY)
@JoinColumn(name="audit_factory_id")
private AuditFactory auditFactory;
@Column(name = "report_id")
private String  reportId;有A页面 
<form:form commandName="auditFactory" action="updateAuditFactory.do" name="auditFactory"
method="post">
<table>
<tr>
<td><label>Factory name:</label></td>
<td><form:input path="factoryName" /><form:hidden path="auditFactoryId" /><br/>
<form:errors path="factoryName" cssStyle="color:red"></form:errors></td>
</tr>
<tr>
<td><label>RBO:</label></td>
<td><form:select path="factoryRetailer">
<c:forEach items="${retailers}" var="retailer">
<form:option value="${retailer}"></form:option>
</c:forEach>
</form:select></td>
</tr>
<tr>
<td><label>Country:</label></td>
<td><form:select path="Country">
<c:forEach items="${countries}" var="country">
<form:option value="${country.name}"></form:option>
</c:forEach>
</form:select></td>
</tr>
<tr>
<td><label>Next audit date:</label></td>
<td><form:input path="auditNextDate" id="datepicker"/><br/>
<form:errors path="auditNextDate" cssStyle="color:red"></form:errors>
</td>
</tr>
<tr>
<td><label>Interval time:</label></td>
<td><form:input path="interval" id="interval"/><br/>
<form:errors path="interval" cssStyle="color:red"></form:errors></td>
</tr>
<tr>
<td><label>Report No:</label></td>
<td><input type="text" id="reportId" name= "reportId"/><br/>
</td>
</tr>
<tr>
<td colspan="2"><input type="submit" value="Submit"  /><input
type="button" value="Cancel"
onclick="JAVASCRIPT:go('viewContracts.do')" /></td>
</tr>
</table>
</form:form>
这时页面有 一方的信息，并添加一条多方信息，那么后台更新时，我想根据它的Id 把多方带出来，在多方中加上这条@RequestMapping(value = "/updateAuditFactory")
public String update (HttpServletRequest request,@ModelAttribute("auditFactory")AuditFactory auditFactory,@RequestParam("reportId") String reportId ){
if(!reportId.equals("")){
AuditReport report=new AuditReport();
report.setReportId(reportId);
report.setAuditFactory(auditFactory);
AuditFactory factory=auditFactoryService.get(auditFactory.getAuditFactoryId());
List<AuditReport> factories=factory.getAuditReports();
factories.add(report);
auditFactory.setAuditReports(factories);
auditFactoryService.update(auditFactory);
}
return "audit/success";
}
服务器报错，说session不知道更新哪一个，这怎么办，如何在页面没有多方的情况下，得到多方来更新