4. 如何在保存时更新其他模型中的非规范化字段？

You have models like this.

class Category(models.Model):
    name = models.CharField(max_length=100)
    hero_count = models.PositiveIntegerField()
    villain_count = models.PositiveIntegerField()

    class Meta:
        verbose_name_plural = "Categories"


class Hero(models.Model):
    name = models.CharField(max_length=100)
    category = models.ForeignKey(Category, on_delete=models.CASCADE)
    # ...


class Villain(models.Model):
    name = models.CharField(max_length=100)
    category = models.ForeignKey(Category, on_delete=models.CASCADE)
    # ...

You need the hero_count and villain_count, to be updated when new objects are created.

You can do something like this

class Hero(models.Model):
    # ...

    def save(self, *args, **kwargs):
        if not self.pk:
            Category.objects.filter(pk=self.category_id).update(hero_count=F('hero_count')+1)
        super().save(*args, **kwargs)


class Villain(models.Model):
    # ...

    def save(self, *args, **kwargs):
        if not self.pk:
            Category.objects.filter(pk=self.category_id).update(villain_count=F('villain_count')+1)
        super().save(*args, **kwargs)

Note how we did not use self.category.hero_count += 1, as update will do a DB update.

另一种方法是使用signals。 You can do it like this.

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

@receiver(pre_save, sender=Hero, dispatch_uid="update_hero_count")
def update_hero_count(sender, **kwargs):
    hero = kwargs['instance']
    if hero.pk:
        Category.objects.filter(pk=hero.category_id).update(hero_count=F('hero_count')+1)

@receiver(pre_save, sender=Villain, dispatch_uid="update_villain_count")
def update_villain_count(sender, **kwargs):
    villain = kwargs['instance']
    if villain.pk:
        Category.objects.filter(pk=villain.category_id).update(villain_count=F('villain_count')+1)

4.1. Signals vs Overriding .save

由于.save的任何一个信号都可以用于保存行为，因此什么时候应该使用哪个信号？ I follow a simple rule.

• 如果您的字段取决于您控制的模型，请覆盖.save
• 如果您的字段依赖于您无法控制的第三方应用程序中的模型，请使用信号。