如何在Django模型中访问外键值？

Question

我正在尝试构建一个简单的“队列”应用程序，在该应用程序中，用户可以在某个房间的队列中用数字取票。

我在定义Ticket.number和Room.next_number_to_take之间的关系时遇到了一些问题。 创建票证时，Ticket.number的值应为Room.next_number_to_take，但是，如果不为该类指定初始化函数，我不知道如何用“ Django”语言定义它。

这是我失败的尝试（失败了故障单的save方法）：

from django.db import models
from django.utils import timezone
from django.contrib.auth.models import User


# Create your models here.
class Room(models.Model):
    name = models.CharField(max_length=100)
    room_master = models.ForeignKey(User, on_delete=models.CASCADE)
    current_visitor_number = models.BigIntegerField(default=0)
    next_number_to_take = models.BigIntegerField(default=0)

class Ticket(models.Model):
    request_date = models.DateTimeField(default=timezone.now)
    user = models.ForeignKey(User, on_delete=models.CASCADE)
    room = models.ForeignKey(Room, on_delete=models.CASCADE)
    number = models.BigIntegerField(default=0)

    def save(self, *args, **kwargs):
        self.number = self.room.next_number_to_take
        self.room.next_number_to_take += 1
        super(Ticket, self).save(*args, **kwargs)

我以前尝试过以下同样失败的方法：

class Ticket(models.Model):
    request_date = models.DateTimeField(default=timezone.now)
    user = models.ForeignKey(User, on_delete=models.CASCADE)
    room = models.ForeignKey(Room, on_delete=models.CASCADE)
    number = room.next_number_to_take
    room.next_number_to_take += 1

Answer 1

我自己还没有尝试过此代码。 我认为，当您更改“ Room模型的next_number ，不会保存该模型。 所以我添加了另一行。 看看它是否有效，如果有任何错误，请尽快与我联系。

def save(self, *args, **kwargs):
    self.number = self.room.next_number_to_take
    self.room.next_number_to_take += 1 # this line is ok but you need to save it
    self.room.save() # this is the new line to add
    super(Ticket, self).save(*args, **kwargs)