异常通常发生在如果您加载一个实体具有集合 cascade = all-delete-orphan 的集合中，然后删除集合的引用。

请勿替换此集合。始终使用 collection.clear（）删除所有关联的子条目，以便孤儿删除算法可以检测到这些更改。如果你想删除任何特定的孩子，你只需要从集合中删除它。一旦从收藏中删除，它将被视为孤儿，并将被删除。

I am getting an error:

Don't change the reference to a collection with cascade="all-delete-orphan"

while trying the following operation:

beginTx();
Parent parent = new Parent();
Child child = new Child();
parent.addChild(child);
getSession().save(parent);
commitTx();
closeSession();

beginTx();
//id is the primary key
child.setID(null);
getSession().update(child);
commitTx();
closeSession();

Parent and child are related by one-to-many with cascade = 'all-delete-orphan'.

class Parent {
Set child;
}


<set name="child" table="Child" cascade="all-delete-orphan" inverse="true">
    <key column="FK"></key>
    <one-to-many class="Child"/>
</set>

Any idea why this exception is being thrown? Why is setting null on the primary key causing this exception even though the entity is in detached state ?

This exception normally happens if you load an entity having a collection with cascade=all-delete-orphan, and then you remove the reference to the collection.

Don't replace this collection. Always use collection.clear() to remove all the associated child entries so that the orphan-deletion algorithm can detect the changes. And if you want to remove any particular child, you just need to remove it from the collection. Once it is removed from the collection, it will be considered as an orphan and will be deleted.