Django：如何创建从其他模型计算的模型？

Question

我有费用和收入模型。这两个模型应该是其他模型平衡的基础。

在哪里

balance.income = sum(incomes)
balance.expense = sum(expenses)
balance.rest = sum(incomes) - sum(expenses)

如何在 Django 中实现这种平衡模型？期望是，每当收入和支出模型从用户那里获得新数据时，余额将自动计算。下面是我对这些模型的实现，但是 Balance 模型需要修改（这里只是为了给出关于结构的想法）。

from django.db import models

class User(models.Model):
    user_id = models.AutoField(primary_key=True)
    firstname = models.CharField(max_length=20)
    lastname = models.CharField(max_length=20)

    def __str__(self):
        return self.firstname

class Balance(models.Model):
    user = models.OneToOneField(User, on_delete=models.CASCADE)
    currency = models.CharField(max_length=3, default='$')
    income = models.IntegerField(default=0)
    expense = models.IntegerField(default=0)
    rest = models.IntegerField(default=0)
    saving = models.IntegerField(default=0)

    def __str__(self):
        return str(self.rest)

class Expense(models.Model):
    user = models.ForeignKey(User, on_delete=models.CASCADE)
    date = models.DateField()
    category = models.TextField(default='')
    detail = models.TextField(default='')
    place = models.TextField(default='')
    amount = models.FloatField(default=0.0)

    def __str__(self):
        return self.detail

class Income(models.Model):
    user = models.ForeignKey(User, on_delete=models.CASCADE)
    date = models.DateField()
    detail = models.TextField(default='')
    amount = models.FloatField(default=0.0)

    def __str__(self):
        return self.detail

Answer 1

最简单的方法是重写.save() 方法。请注意，通过命令行保存模型时会绕过这一点。

class Balance(models.Model):
    user = models.OneToOneField(User, on_delete=models.CASCADE)
    currency = models.CharField(max_length=3, default='$')
    income = models.IntegerField(default=0)
    expense = models.IntegerField(default=0)
    rest = models.IntegerField(default=0)
    saving = models.IntegerField(default=0)

    def __str__(self):
        return str(self.rest)

    def save(self, *args, **kwargs):
        user_incomes = Income.objects.filter(user=self.user).aggregate(Sum('amount'))['amount_sum']
        user_expenses = Expense.objects.filter(user=self.user).aggregate(Sum('amount'))['amount_sum']
        self.income = user_incomes
        self.expense = user_expenses
        self.balance = user_incomes - user_expenses
        super(Balance, self).save(*args, **kwargs)