错误中学习--a different object with the same identifier value was already associated with the session

出现如下错误 a different object with the same identifier value was already associated with the session的场景是: 我直接用hql的update(obj)更新一条数据,但是有两个字段因为没有值被更新成了null,所以想在更新前,先把这两个字段查出来赋值给实体类,这样更新的时候可以用到原来的值 我在service

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