Why NHibernate deletes referenced objects one by one, not using foreign key?

Question

I have simple Parent-Child relationship, Parent has many Child objects, the relation is unidirectional:

public class Parent
{
    public virtual int Id { get; protected set; }
    public virtual string Name { get; set; }
    public virtual IList<Child> Children { get; set; }
}

public class Child
{
    public virtual int Id { get; protected set; }
    public virtual string Name { get; set; }
}

Mapping for the relation sets cascade to AllDeleteOrphan to remove Child objects not referenced by Parent any more:

HasMany(x => x.Children).Cascade.AllDeleteOrphan();

And now I'm clearing the list of Child objects:

var parent = session.Get<Parent>(1);
parent.Children.Clear();
session.Update(parent);

NHibernate deletes the Child object as expected, but it do this by sending separate DELETE query for each Child from the collection: DELETE FROM Child WHERE Id = ... - that can mean really LOT of queries.

Anyway, it can be easily done using single query like DELETE FROM Child WHERE ParentId = 1. Why NHibernate is not using the parent foreign key to clear the collection? It seems that it knows everything to prepare such query (which foreign key, what value etc.)?

Answer 1

NHibernate can and will do that in certain cases.

It's all explained here.