Entities:

public class Parent
{
    virtual public long Id { get; set; }
    virtual public string Description { get; set; }

    virtual public ICollection<Child> Children { get; set; }
}

public class Child
{
    virtual public long Id { get; set; }
    virtual public string Description { get; set; }

    virtual public Parent Parent { get; set; }
}

Mappings:

public class ParentMap : ClassMap<Parent>
{
    public ParentMap()
    {
        Id(x => x.Id).GeneratedBy.Assigned();

        Map(x => x.Description);

        HasMany(x => x.Children)
            .AsSet()
            .Inverse()
            .Cascade.AllDeleteOrphan();
    }
}

public class ChildMap : ClassMap<Child>
{
    public ChildMap()
    {
        Id(x => x.Id).GeneratedBy.Assigned();

        Map(x => x.Description);

        References(x => x.Parent)
            .Not.Nullable()
            .Cascade.All();
    }
}

and

        using (var session = sessionFactory.OpenSession())
        using (var transaction = session.BeginTransaction())
        {
            var parent = new Parent { Id = 1 };

            parent.Children = new HashSet<Child>();

            var child1 = new Child { Id = 2, Parent = parent };
            var child2 = new Child { Id = 3, Parent = parent };

            parent.Children.Add(child1);
            parent.Children.Add(child2);

            session.Save(parent);

            transaction.Commit();
        }

this codes generates following sql

NHibernate: SELECT child_.Id, child_.Description as Descript2_0_, child_.Parent_id as Parent3_0_ FROM [Child] child_ WHERE child_.Id=@p0;@p0 = 2 [Type: Int64 (0)]
NHibernate: SELECT child_.Id, child_.Description as Descript2_0_, child_.Parent_id as Parent3_0_ FROM [Child] child_ WHERE child_.Id=@p0;@p0 = 3 [Type: Int64 (0)]
NHibernate: INSERT INTO [Parent] (Description, Id) VALUES (@p0, @p1);@p0 = NULL[Type: String (4000)], @p1 = 1 [Type: Int64 (0)]
NHibernate: INSERT INTO [Child] (Description, Parent_id, Id) VALUES (@p0, @p1, @p2);@p0 = NULL [Type: String (4000)], @p1 = 1 [Type: Int64 (0)], @p2 = 2 [Type:Int64 (0)]
NHibernate: INSERT INTO [Child] (Description, Parent_id, Id) VALUES (@p0, @p1, @p2);@p0 = NULL [Type: String (4000)], @p1 = 1 [Type: Int64 (0)], @p2 = 3 [Type:Int64 (0)]

Why are these two selects generated and how can I remove it?