Is saving an entity with a component collection supposed to do a SQL DELETE then INSERT using NHibernate?

Question

I have an entity, TrackLog that has a component collection of TrackPoints. I create a new TrackLog and add some Trackpoints to it and save it to the DB.

When I grab my TrackLog again and add a new TrackPoint, instead of doing one SQL INSERT like I expect, it's doing a DELETE on all the TrackPoints associated with the TrackLog and then re-adding every TrackPoint that's in the collection.

This is the output from NHProf:

UPDATE TrackLog SET Version = 2 etc...
DELETE FROM TrackPoints WHERE TrackLog_Id = '80cc1e0d' etc...
INSERT INTO TrackPoints etc..
INSERT INTO TrackPoints etc...
INSERT INTO TrackPoints etc...

My question is, is this normal behavior for a component collection? This happens as well for my enum collections. If this is not normal behavior, how do I modify my mappings so that it exhibits the correct behavior (only one INSERT for the TrackPoint I added)?

Answer 1

It is normal.

To avoid it, declare your collection as a set instead of a bag (and use compatible types) if your use case supports it (i.e. there can't be two trackpoints with the same data)