how does nhibernate handle update collisions behind the scenes with the ver column?

I'm just looking for some documentation on how hibernate handles collisions using the ver column.

understanding thus far: a change to a row means that the changed row is inserted with a new version number

-- Is this done via "select max(ver)+1"? --

subsequently the previous version gets deleted.

btw: how do we know that {"sel开发者_JS百科ect max(ver)+1"; insert} will be guaranteed to be an atomic operation? Are they locking the row first?

Thanks in advance for any info. I'm just trying to gain a complete understanding.

I'll expand a bit more here. What hibernate executes for an update is something like

UPDATE table SET string = 'string', number = 1, version = version+1 
WHERE id = %Id% AND version = %Version%

This means that the update will only succeed if the version is still the same as when the object/row was loaded from the database and update the version number in the same query.

See http://ayende.com/blog/3946/nhibernate-mapping-concurrency