save returns the id and pk none

[fabricio]

my model


class Caixafaccao(models.Model):

    empresa = models.ForeignKey(Empresa, null=True, blank=True)
    cliente = models.ForeignKey(Cliente, verbose_name=u'Cliente')
    conta = models.ForeignKey(Conta, verbose_name=u'Conta' )
    subconta = models.ForeignKey(SubConta, verbose_name=u'Sub-Conta' )
    caixa = models.IntegerField(u'Caixa', blank=True, null=True)
    entrega = models.IntegerField(u'Entrega', blank=True, null=True)
    entregaitens = models.IntegerField(u'Entrega Itens', blank=True, null=True)
    dtlancamento = models.DateField(u'Data lançamento', default=datetime.now, null=True, blank=True)
    idcentrocusto = models.CharField(u'Centro de custo', max_length=1, default='', choices=Centro_custo_CHOICE)
    valor = models.DecimalField(u'Vlr. Unit.', max_digits=18, decimal_places=2, default=0)
    tipo = models.CharField(u'Tipo', max_length=1, default='', null=True, blank=True, choices=DebCred_CHOICE)
    observacoes = models.TextField(u'Observações', max_length=650, default='', null=True, blank=True)
    valordefeito = models.DecimalField(u'Vlr. Defeito.', default=0, max_digits=18, decimal_places=2, null=True, blank=True)


    class Meta:
        ordering = ['dtlancamento','id' ]

    def __unicode__(self):
       return u'%s' % (self.id)






my form


class CaixafaccaoForm(ModelForm):
    class Meta:
        model = Caixafaccao

    def __init__(self, *args, **kwargs):
        super( CaixafaccaoForm , self).__init__(*args, **kwargs)

        self.fields["dtlancamento"].widget.attrs['readonly'] = "True"
        self.fields["dtlancamento"].widget.attrs['class'] = "data"
        self.fields["cliente"].widget.attrs['class'] = "chosen-select-no-results"
        self.fields["observacoes"].widget.attrs['style'] = "width: 555px; height: 150px; resize: none;"
        self.fields["conta"].widget.attrs['class'] = "chosen-select-no-results"
        self.fields["subconta"].widget.attrs['class'] = "chosen-select-no-results"


    def __new__(cls, *args, **kwargs):
        new_class = super(CaixafaccaoForm, cls).__new__(cls, *args, **kwargs)
        for field in new_class.base_fields.values():
            if isinstance(field, DecimalField):
                field.localize = True
                field.widget.is_localized = True
        return new_class





My view code

 if request.method == 'POST':
            form_caixafaccao = CaixafaccaoForm(request.POST )
            if form_caixafaccao.is_valid():

                caixafaccao = form_caixafaccao.save( commit=False )



when I get the form_caixafaccao.save caixafaccao returns with id = PK = None and None debugging and saw that has an error DoesNotExist <class'movimento.models.DoesNotExist'> if someone can help me thank you.