Django模型中的自定义函数

Question

如何使用现有模型字段计算总数

class Book(models.Model):
    school_id = models.ForeignKey(School)
    name = models.CharField(max_length=100, default=None, blank=None, 
unique=True)
    class_name = models.CharField(max_length=50)
    category = models.ForeignKey(Category)
    bundle = models.CharField(max_length=20, unique=True)
    particulars = models.CharField(max_length=50)
    tax_code = models.CharField(max_length=50, default=None)
    amount = models.FloatField()
    tax_CGST = models.FloatField(default=0)
    tax_SGST = models.FloatField(default=0)
    total = models.FloatField()

    def total(self):
        return ((self.tax_SGST+self.tax_CGST)*self.amount)/100

    def __str__(self):
        return self.name

在上面的代码中，我希望total函数从Tax和amount字段计算总数，并将其添加到数据库中的total字段