使用Django的Case/When查询集条件来设置多个值？

from django.db import models
from django.db.models import Case, When, Value

class User(models.Model):
    age = models.IntegerField()
    gender = models.CharField(max_length=10)
    category = models.CharField(max_length=20, null=True)

    @classmethod
    def set_category(cls):
        cls.objects.update(
            category=Case(
                When(age__lt=18, gender='male', then=Value('Young Male')),
                When(age__lt=18, gender='female', then=Value('Young Female')),
                When(age__gte=18, gender='male', then=Value('Adult Male')),
                When(age__gte=18, gender='female', then=Value('Adult Female')),
                default=Value('Unknown')
            )
        )

User.set_category()