HasMany() Collection of concrete Subclass

Hello, I have such business model:

table A (table per class)
ID
TYPE
REF_ID

abstract class A
{
ObjType { get; set; }
...
}
// ObjType = 'B'
class B : A
{
...
}
// ObjType = 'C'
class C : A
{
...
}
class D
{
IList<B> childs { get; set; }
...
}

Mapping for class A:
DiscriminateSubClassesOnColumn("TYPE");

Mapping for class B:
DiscriminatorValue("B")

Mapping for class C:
DiscriminatorValue("C")

Mapping for class D:
HasMany(x => x.childs)
                .KeyColumn("REF_ID")
                .Inverse()
                .Cascade.AllDeleteOrphan()
                .LazyLoad();

The problem is that sql for collection "childs" is generated with out condition on field "TYPE", something like that:
select * from A where ref_id = :param

But if I do session.Get<B>(id) everything goes fine.

I know it is possible to add "Where" clause to "HasMany" but it is not good idea on my mind.

Do someone the solution for this issue?