How to save a cascaded collection in nhibernate without putting circular reference?

Always when I try to save an object with a collection of data mapped as HasMany I have to put a circular reference so that NHibernate can save this object cascade.

For example I have the following mapping of the parent class.

public class ClasseMap : ClassMap<Classe>
{
    public ClasseMap()
    {
      Table("tClasse");

      HasMany<Metodo>(t => t.Metodos)
     .KeyColumn("IdClasse")
     .Inverse()
     .Cascade
     .SaveUpdate();
    }
}

And in the child class

public class MetodoMap : ClassMap<Metodo>
{
    public MetodoMap()
    {
      Table("tMetodo");

      References(t => t.Classe)
            .Column("IdClasse")
            .ForeignKey("FK_Metodo_IdClasse");
    }
}

When I save the data from my object Classe I do the following:

public class RepositorioClasse
{
    protected readonly ISession SessaoAtual;

    public RepositorioClasse(ISession sessaoAtual)
    {
        SessaoAtual = sessaoAtual;
    }

    public SalvaEntidades() {
        var classe = new Classe();
        var metodo = new Metodo(){
                           Nome= "ToString",
                           TipoRetorno="String",
                           Classe= classe
                          };    

        classe.Nome = "String";
        classe.Metodos.Add(metodo);

        SessaoAtual.SaveUpdate(classe);
   }
}

If I only put a collection of Metodos into Classe and remove the reference from Classe to Metodo NHibernate does not save and returns an exception saying that I should have a reference of Classe in Metodo .