Django更新/查询

Question

我试图更新模型中的变量，但它没有发生。不知道为什么？我之前也一直这样做过去曾经工作过，但不知道为什么不发生？任何建议。

我的模特

class VehicleDetails(models.Model):
    vehicle_name = models.CharField(max_length = 255)
    total_shipments = models.CharField(max_length = 255)
    remaining_shipments = models.CharField(max_length = 255)
    status = models.CharField(max_length = 255)
    time = models.DateTimeField( auto_now_add = True, db_index = True)

我想做的事

        vehicle_details_obj = VehicleDetails.objects.filter(status = 'open').order_by('-time')
        try: 
            vehicle_name = vehicle_details_obj[0].vehicle_name
            remaining_shipments = int(vehicle_details_obj[0].remaining_shipments) - 1
            print remaining_shipments
            vehicle_details_obj[0].remaining_shipments = str(remaining_shipments)
            if remaining_shipments == 0: vehicle_details_obj[0].status = 'close'
            vehicle_details_obj[0].save()
        except : 
            vehicle_name = 'NA'

我通过从中减去1来更新remaining_shipments字段，然后更新它。我之前曾经这样更新，但它没有发生。我也试过了vehicle_details_obj[0].update(remaining_shipments = remaining_shipments)，但它不起作用。任何帮助都感激不尽。感谢