How do I save a new object using NHibernate when it already has an ID?

I'm using GUIDs for the identifier on my objects it is assigned when the 开发者_如何学JAVAentity is instantiated. I want to save them using NHibernate and have NHibernate determine whether it's a new object or not. If it's a new object, it should INSERT it, otherwise it should UPDATE it.

How would I do this?

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You can do that by adding either version or timestamp to your entity.

Without one of those it's impossible for Hibernate to figure out whether your entity should be inserted or updated since you're using an assigned identifier.