如何重写django save方法来动态更新某些字段？

Question

这里我有两个模型。在这些模型中，我想amount_to_pay在Ledger模型中实现动态值。例如，对于这两个模型，我有两种不同的形式，并且如果用户选择payment_option来自ledger模型的模型并给出一些值，则可以节省费用形式amount_to_pay那么如果仅该字段ledger.id并且与该字段expense.payment_option_id相同，amount_to_pay则应将账本模型中的值替换为该值。我该怎么办？

models.py

     class Expense(models.Model):
        pay_from = models.CharField(max_length=200)
        payment_option = models.ForeignKey('Ledger', on_delete=models.CASCADE)
        amount_to_pay = models.IntegerField(default=0)
        expense_date = models.DateField(default=datetime.date.today)
        expense_type = models.ForeignKey(ExpenseType, on_delete=models.CASCADE)
        note = models.TextField()
        created = models.DateTimeField(auto_now_add=True)
        updated = models.DateTimeField(auto_now=True)
        slug = AutoSlugField(unique_with='id', populate_from='expense_type')

        def get_amount_to_pay(self):
            return self.amount_to_pay


    class Ledger(models.Model):
        name = models.CharField(max_length=200)
        account_number = models.CharField(max_length=250, unique=True)
        account_type = models.CharField(max_length=200)
        opening_balance = models.IntegerField(default=0)
        amount_to_pay = models.IntegerField(default=0, blank=True, null=True)
        current_balance = models.IntegerField(default=0, blank=True, null=True)
        created = models.DateTimeField(auto_now_add=True)
        updated = models.DateTimeField(auto_now=True)
        slug = AutoSlugField(unique_with='id', populate_from='name')

        def save(self, *args, **kwargs):
             self.amount_to_pay = Expense.get_amount_to_pay(self)

# here how can i save the amount_to_pay from expense form if the ledger.id and expense.payment_option.id matches??
#i got stuck here.           
 self.current_balance = self.opening_balance - self.amount_to_pay

            super(Ledger, self).save(*args, **kwargs)

Answer 1

解决方案一：

我认为Ledger您应该更改Expense模型，而不是更改模型，如下所示：

class Expense(models.Model):
    ...
    def save(self, *args, **kwargs):
       self.payment_option.amount_to_pay = self.payment_option.amount_to_pay + self.amount_to_pay
       self.payment_option.save()
       super(Expense, self).save(*args, **kwargs)

解决方案二：

但老实说，解决方案一对我来说似乎并不好。原因是您将相同的数据保存在2个地方（费用和分类帐中）。相反，它应该是一次，然后应动态计算amount_to_payin 的值Ledger。像这样：

from django.db.models import Sum

class Ledger(...):

     @property
     def amount_to_pay(self):
         # I am using a property method to show the amount_to_pay value.
         # FYI: in this way, you need to remove amount_to_pay field from Ledger model
         return self.opening_balance - self.expense_set.all().aggregate(a_sum=Sum('amount_to_pay')).get('a_sum', 0)

这样，对于每个分类帐，amount_to_pay将在运行时动态计算值。例如：

 for l in Ledger.objects.all():
     l.amount_to_pay

解决方案三：

如果您不愿使用l.amount_to_pay先前的解决方案来使数据库命中（因为它会动态地从数据库计算出amount_to_pay），那么您可以随时使用annotate该值。像这样：

对于此解决方案，您需要更改Expense模型并添加related_name：

class Expense(models.Model):
    pay_from = models.CharField(max_length=200)
    payment_option = models.ForeignKey('Ledger', on_delete=models.CASCADE, related_name='expenses')

然后related_name在这样的查询中使用它（仅供参考：对于以下用法示例，您无法def amount_to_pay(...)在Ledger模型中保留方法）：

from django.db.models import Sum, F, ExpressionWrapper, IntegerField

ledgers = Ledger.objects.all().annotate(expense_sum=Sum('expenses__amount_to_pay')).annotate(amount_to_pay=ExpressionWrapper(F('opening_balance') - F('expense_sum'), output_field=IntegerField()))

# usage one
for l in ledgers:
   l.amount_to_pay

# usage two
ledgers.values('amount_to_pay')