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"NHibernate.NonUniqueObjectException: a different object with the same identifier value was already associated with the session"

I’m a big fan of NHibernate(if I didn’t mention this before). The only problem is that it’s error messages can be kind of cryptic, especially if you have limited knowledge about NHibernate.

A colleague came to me with the following problem: He tried to update a detached object that he got back from an API call and constructed himself, NHibernate refused to save it(using the Update method) and returned the following error message:

"NHibernate.NonUniqueObjectException: a different object with the same identifier value was already associated with the session"

The problem was that a similar object(using the same identifier) was already loaded and tracked by the session. When he tried to associate the detached object to the session, NHibernate recognizes a similar object with the same identifier and raises the error above.

To fix this, you have to switch from Update() to Merge(). This method copies the state of the given object onto the persistent object with the same identifier. If there is no persistent instance currently associated with the session, it will be loaded. The method returns the persistent instance. If the given instance is unsaved or does not exist in the database, NHibernate will save it and return it as a newly persistent instance.

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