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NHibernate–Persist vs Save

While reviewing some code today I noticed that a developer was using the NHibernate session.PersistAsyc() method to save an entity in the database. I always used the session.SaveAsync() method so I was kinda intrigued about the difference between the two.

Let’s first talk about how they are the same; both methods make a transient instance persistent. This means that a new object created somewhere inside your code will be inserted into the database using an INSERT statement.

The picture below explains the different states of an NHibernate object quite well:

nhibernatestates

The differences

Ok, now we know in which way they are the same. How are they different then?

  • The SaveAsync() method returns the persisted object whereas the PersistAsync() method returns nothing.
  • The PersistAsync() method doesn't guarantee that the identifier value will be assigned to the persistent instance immediately, the assignment might happen at flush time. This means that when calling the SaveAsync() method, the INSERT operation is executed immediatelly.
  • Calling SaveAsync() on a detached entity will result in a new INSERT statement whereas calling PersistAsync will throw an exception.
  • Calling PersistAsync() outside of transaction boundaries will do nothing whereas doing the same using SaveAsync will result in the execution of an INSERT statement.

If you know more differences, please feel free to share…

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