I have two entities in a bi-directional one-to-many relationship:

public class Storage
{
    public IList<Box> Boxes { get; set; }
}

public class Box
{
    public Storage CurrentStorage { get; set; }
}

And the mapping:

<class name="Storage">
    <bag name="Boxes" cascade="all-delete-orphan" inverse="true">
        <key column="Storage_Id" />
        <one-to-many class="Box" />
    </bag>
</class>

<class name="Box">
    <many-to-one name="CurrentStorage" column="Storage_Id" />
</class>

A Storage can have many Boxes, but a Box can only belong to one Storage. I have them mapped so that the one-to-many has a cascade of all-delete-orphan.

My problem arises when I try to change a Box's Storage. Assuming I already ran this code:

var storage1 = new Storage();
var storage2 = new Storage();
storage1.Boxes.Add(new Box());

Session.Create(storage1);
Session.Create(storage2);

The following code will give me an exception:

// get the first and only box in the DB
var existingBox = Database.GetBox().First();

// remove the box from storage1
existingBox.CurrentStorage.Boxes.Remove(existingBox);

// add the box to storage2 after it's been removed from storage1
var storage2 = Database.GetStorage().Second();
storage2.Boxes.Add(existingBox);

Session.Flush(); // commit changes to DB

I get the following exception:

NHibernate.ObjectDeletedException : deleted object would be re-saved by cascade (remove deleted object from associations)

This exception occurs because I have the cascade set to all-delete-orphan. The first Storage detected that I removed the Box from its collection and marks it for deletion. However, when I added it to the second Storage (in the same session), it attempts to save the box again and the ObjectDeletedException is thrown.

My question is, how do I get the Box to change its parent Storage without encountering this exception? I know one possible solution is to change the cascade to just all, but then I lose the ability to have NHibernate automatically delete a Box by simply removing it from a Storage and not re-associating it with another one. Or is this the only way to do it and I have to manually call Session.Delete on the box in order to remove it?