如何在Django models类中实现多变量同值赋值 同步weight到amount

你现有的amount = models.FloatField(weight)写法不符合Django的字段定义规则，不能直接将另一个模型字段作为参数传入字段构造函数，要实现需求可以根据你的使用场景选两种方案：

方案1：amount作为可独立修改的存储字段 #

如果你需要amount默认取weight的值，后续可以单独修改，不需要和weight永久绑定，直接重写模型的save方法即可：

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

class Orders(models.Model):
    # 注意原写法primary_key='consignment_id'有误，该参数为布尔值，直接传True即可
    consignment_id = models.IntegerField(primary_key=True, auto_created=True)
    order_date = models.DateTimeField(default=timezone.now)
    weight = models.FloatField()
    from_place = models.CharField(max_length=20, null=True)
    destination = models.CharField(max_length=20)
    # 先允许空值，save方法里再赋值
    amount = models.FloatField(null=True, blank=True)
    name = models.ForeignKey(User, on_delete=models.CASCADE)
    status = models.CharField(max_length=20, default='Pending')

    def save(self, *args, **kwargs):
        # 仅当amount未手动赋值时，用weight填充
        if self.amount is None:
            self.amount = self.weight
        super().save(*args, **kwargs)

方案2：amount和weight永久同步，不单独修改 #

如果你不需要单独修改amount，要求它永远和weight的值保持一致，完全没必要额外存储amount字段，用Python属性动态返回weight的值即可，还能减少数据库冗余：

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

class Orders(models.Model):
    consignment_id = models.IntegerField(primary_key=True, auto_created=True)
    order_date = models.DateTimeField(default=timezone.now)
    weight = models.FloatField()
    from_place = models.CharField(max_length=20, null=True)
    destination = models.CharField(max_length=20)
    name = models.ForeignKey(User, on_delete=models.CASCADE)
    status = models.CharField(max_length=20, default='Pending')

    # 动态生成amount属性，调用时直接返回weight的值
    @property
    def amount(self):
        return self.weight

如果需要在数据库查询阶段使用amount做过滤、聚合操作，直接用F('weight')表达式代替F('amount')即可，效果完全一致。

内容的提问来源于stack exchange，提问作者user17489899