...
- All -> Composite
- All-Delete-Orphan -> Composite
- Others -> Aggregation
Customizing nhibernate deletes
NHibernate still does cascade on relationships and this can be a problem. For example when a object is reparented. In the sample below object A was a part-of (composition) D1 but it moved to D2. All relationships below are composite. Now if the cascade of the mapping between D1 and A is all NHibernate will delete A because D1 is deleted. This is obviously not what we want because D2 uses A now. This will result in a 'deleted object will be resaved by cascade' exception. To prevent this we need to intercept Nhibernate's deletes and verify that the object really should be deleted. This can be done by checking if the object that NH wants to delete really is a orphan. 
Further reading
Fowler about aggregate vs composition :http://martinfowler.com/bliki/AggregationAndComposition.html
NHibernate cascades : http://ayende.com/blog/1890/nhibernate-cascades-the-different-between-all-all-delete-orphans-and-save-update