NHibernate cascade collection delete when insert new item to not empty collection - nhibernate

Question

I have the following Fluent Mappings:
public ScanDeliverySessionMap()
{
Id(x => x.Id);
...
...
HasManyToMany(x => x.ToScanForms) <--- IList<Form> ToScanForms --->
.Table("ToScanForm")
.ParentKeyColumn("SessionId")
.ChildKeyColumn("FormId").Cascade.SaveUpdate();
}
public FormMap()
{
Id(x => x.Id).Column("FormID").GeneratedBy.Foreign("Log");
....
....
HasManyToMany(x => x.ScanDeliverySessions)
.Table("ToScanForm")
.ParentKeyColumn("FormId")
.ChildKeyColumn("SessionId").Inverse();
}
When I try to insert new Form to the ToScanForms collection
Everything seemingly works properly but watching on NHProf
I see that NH casacde DELETE over all ToScanForms items
and then NH INSERT the ToScanForms items including the new item.
Some screenshots:

Answer 1

That behaviour occurs because nhibernate doesn't know which entities in the collection are new and which are old, so he must delete everything and then re-insert them.
To prevent this is behaviour is quite simple: change your property to an ICollection and map your HasManyToMany as a set. You mapping would be changed to the following:
public ScanDeliverySessionMap()
{
Id(x => x.Id);
...
...
HasManyToMany(x => x.ToScanForms) //<--- ICollection<Form> ToScanForms --->
.AsSet()
.Table("ToScanForm")
.ParentKeyColumn("SessionId")
.ChildKeyColumn("FormId").Cascade.SaveUpdate();
}
public FormMap()
{
Id(x => x.Id).Column("FormID").GeneratedBy.Foreign("Log");
....
....
HasManyToMany(x => x.ScanDeliverySessions)
.AsSet()
.Table("ToScanForm")
.ParentKeyColumn("FormId")
.ChildKeyColumn("SessionId").Inverse();
}
Under the hood nhibernate will use Iesi Collections' HashSet, so now he knows which entities are new and which are old.