Hibernate级联保存，父表为空，先存子表，保存提示外键为null的错误怎么解决？
两张表：父表people，子表picture
people的映射文件如下，省略字段的描述

<hibernate-mapping package="com.myproject.entity">

	<class name="People" table="PEOPLE">

		<meta attribute="sync-DAO">false</meta>

		<cache usage="read-write"/>

		<id name="id" type="java.lang.Long" column="PEOPLE_ID">

			<generator class="sequence">

				<param name="sequence">S_PEOPLE</param>

			</generator>

		</id>

		<set name="pictures" inverse="true">

			<key column="PEOPLE_ID"></key>

			<one-to-many class="Picture"/>

		</set>

	</class>

</hibernate-mapping>

picture的映射文件如下，省略字段的描述

<hibernate-mapping package="com.myproject.entity">

	<class name="Picture" table="PICTURE">

		<meta attribute="sync-DAO">false</meta>

		<cache usage="read-write"/>

		<id name="id" type="java.lang.Long" column="PICTURE_ID">

			<generator class="sequence">

				<param name="sequence">S_PICTURE</param>

			</generator>

		</id>

		<many-to-one name="people" column="PEOPLE_ID" class="People" not-null="false"></many-to-one>

	</class>

</hibernate-mapping>

我的应用要求是在添加people人物信息的界面，弹出另外一个页面添加人物的picture信息，只有这个页面操作完关闭后，才能返回people添加界面继续操作。
这里在保存picture的时候people还没有生成，people_id就为空了。想法是先保存picture信息，people_id留空，后面保存people信息的时候再更新picture的people_id字段。
但在保存picture的时候报错，保存代码如下：

bean.setTitle(title);

bean.setAddTime(now());

save(bean);

错误提示：

2013-01-17 12:16 INFO  c.j.d.a.f.PictureAction - savePicture()

2013-01-17 12:16 WARN  c.o.x.i.ExceptionMappingInterceptor - null

java.lang.NullPointerException

	at com.myproject.common.hibernate3.BaseManagerImpl.save(BaseManagerImpl.java:125) [BaseManagerImpl.class:na]

	at com.myproject.manager.impl.PictureMngImpl.savePicture(PictureMngImpl.java:40) [PictureMngImpl.class:na]

	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) [na:1.5.0_11]

	at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source) [na:1.5.0_11]

	at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source) [na:1.5.0_11]

	at java.lang.reflect.Method.invoke(Unknown Source) [na:1.5.0_11]

	at org.springframework.aop.support.AopUtils.invokeJoinpointUsingReflection(AopUtils.java:307) [spring-aop-2.5.6.jar:2.5.6]

	at org.springframework.aop.framework.ReflectiveMethodInvocation.invokeJoinpoint(ReflectiveMethodInvocation.java:182) [spring-aop-2.5.6.jar:2.5.6]

	at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:149) [spring-aop-2.5.6.jar:2.5.6]

	at org.springframework.transaction.interceptor.TransactionInterceptor.invoke(TransactionInterceptor.java:106) [spring-tx-2.5.6.jar:2.5.6]

	at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:171) [spring-aop-2.5.6.jar:2.5.6]

	at org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:204) [spring-aop-