Django ORM：将模型之间的外键关系限制为有限的次数

Question

我有两种模型：车辆和驾驶员。在任何给定时间点，一辆车可以有零个、一个或最多两个司机。我当前的模型设置允许在一辆车上拥有超过 2 名司机。如何将司机人数限制为每辆车只有两名？

我目前的模型

class DriverStatus(models.Model):
status = models.CharField(max_length=200)
def __str__(self):
    return f'{self.status}'



class Driver(models.Model):
    name = models.CharField(max_length=200)
    driver_num = models.IntegerField()
    profile_image = models.ImageField(default="default.jpg",upload_to="images")
    status = models.ForeignKey(DriverStatus, on_delete=models.CASCADE)
    current_vehicle = models.ForeignKey(Vehicle, on_delete=models.SET_NULL,null=True)
    created_at = models.DateTimeField(auto_now_add=True)
    updated_at = models.DateTimeField(auto_now=True)
    def __str__(self):
        return f'{self.name}'


class Vehicle(models.Model):
    vehicle_number = models.CharField(max_length=200)
    created_at = models.DateTimeField(auto_now_add=True)
    updated_at = models.DateTimeField(auto_now=True)
    def __str__(self):
        return f'{self.vehicle_number

Answer 1

您无法在数据库级别执行此操作。但是可以添加模型验证：https://docs.djangoproject.com/en/3.0/ref/validators/

或者您可以在模型的 clean 方法中进行验证： Validate count() constraint in many to many relation before saving an instance in django

---

在 Driver 类中添加：

  def clean(self):
    cleaned_data = super(Driver, self).clean()
    vehicle = self.cleaned_data.get('current_vehicle')
    if not vehicle:
        return cleaned_data
    drivers_count = Driver.objects.filter(current_vehicle=vehicle).exclude(pk=self.instance.pk).count()
    if drivers_count >= 2:
      raise ValidationError('Maximum drivers reached')
    return cleaned_data

我没有测试过，但就是这样！