def METHOD_NAME(self):
    # Make the invoice in a different period for having negative result
    purchase = self._invoice_purchase_create("2014-01-01")
    self._invoice_refund(purchase, "2015-01-18")
    self.model115 = self._create_model_115()
    self.model115.button_calculate()
    self.assertEqual(self.model115.tipo_declaracion, "N")
    self.assertEqual(self.model115.tipo_declaracion_negativa, "N")
    self.assertFalse(self.model115.tipo_declaracion_positiva)
    with self.assertRaises(exceptions.ValidationError):
        self.model115.tipo_declaracion = "I"
    # this doesn't rise any error
    self.model115.tipo_declaracion_negativa = "N"