创建新对象时，如何更新外键上的字段？

2024-05-15 • 问答

我对Django还是很陌生，并且对模型有疑问。

class RestaurantAvailability(models.Model):
    """RestaurantAvailability Model"""
    DayTime = models.DateTimeField()
    restaurant = models.ForeignKey(Restaurant,on_delete=models.CASCADE)
    availability = models.IntegerField(null=True)

    def __str__(self):
        """What to show when we output an object as a string."""
        restaurant = self.restaurant.name
        time = self.DayTime
        return restaurant + " " + str(time)


class Reservation(models.Model):
    """Reservation Model"""
    name=models.CharField(max_length=100)
    email=models.CharField(max_length=100)
    note=models.CharField(max_length=100)
    restaurant = models.ForeignKey(Restaurant,on_delete=models.CASCADE)
    restaurantAvailability = models.ForeignKey(RestaurantAvailability,on_delete=models.CASCADE)

    def __str__(self):
        """What to show when we output an object as a string."""
        person = self.name
        place = self.restaurant.name
        time = self.restaurantAvailability.DayTime
        return person + "  " + place + " " + str(time)

    def save(self,*args,**kwargs):
        """decrement resturant avalability by 1 on create"""
        if not self.pk:
            print('|||||||||||||||||||||||||||||||||||||||||')
            print(self.restaurantAvailability.availability)
            print('|||||||||||||||||||||||||||||||||||||||||')
            self.restaurantAvailability.availability -= 1 #  decrement resturant avalability by 1 on create
            print(self.restaurantAvailability.availability)
        super(Reservation,self).save(*args,**kwargs)

当我创建一个新的预留时，我想将Resturant的可用性递减1。之所以运行save方法是因为我成功创建了一个新的保留，并且甚至在递减后也打印出了正确的可用性。但是，当我进入管理面板查看RestaurantAvailability时，可用性没有任何变化。好像没有保存可用性。

我的问题是创建新对象时如何更新外键上的字段？

更改可用性后，需要保存可用性。注意self.restaurantAvailability.save()行。您也可以执行update语句而不是调用.save()或将该逻辑移至信号。

def save(self,*args,**kwargs):
        """decrement resturant avalability by 1 on create"""
        if not self.pk:
            print('|||||||||||||||||||||||||||||||||||||||||')
            print(self.restaurantAvailability.availability)
            print('|||||||||||||||||||||||||||||||||||||||||')
            self.restaurantAvailability.availability -= 1 #  decrement resturant avalability by 1 on create
            self.restaurantAvailability.save()
            print(self.restaurantAvailability.availability)

额外：可能值得考虑制作transaction atomic，这样您就不会遇到并发请求的问题。

from django.db import models,transaction


@transaction.atomic
def save(self,**kwargs):
    ...

您应该使用post_save signal handler处理此问题。

from django.db.models.signals import post_save
from django.dispatch import receiver


@receiver(post_save,sender=Reservation,dispatch_uid="update_availability")
def update_stock(sender,instance,**kwargs):
    RestaurantAvailability.objects.filter(pk=instance.restaurantAvailability_id).update(availability=F('availability') - 1)

您仍然需要担心并发请求产生的预留量超出处理能力。

创建新对象时，如何更新外键上的字段？

huntliu 回答：创建新对象时，如何更新外键上的字段？

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