org.springframework.orm.hibernate3.HibernateSystemException: illegally attempted to associate a prox

异常描述 

org.springframework.orm.hibernate3.HibernateSystemException: illegally attempted to associate a proxy with two open Sessions; nested exception is org.hibernate.HibernateException: illegally attempted to associate a proxy with two open Sessions

在保存实体对象的时候，提示这个。

可能的原因：

实体对象中与之关联的对象赋值为空导致。

TrainProjectUser tpu = (TrainProjectUser) list.get(i);

SysUser sysUser = tpu.getSysUser(); //为空，这样不行

SysUser sysUser = userManager.findUser(tpu.getSysUser().getId()); 重新取出对象可以

eOrder.setSysUser(sysUser);

orderManager.saveOrder(eOrder);