如何在多对多字段中使用 count()

Question

我的 django 项目是关于资金管理的，所以它的一种资金交易类型是 MultipleTransaction，它只说它是由多个用户共同完成的。

我想在我的界面中显示一个 MultipleTransaction 的总值，使用 Django 模板引擎，我该怎么做？

我尝试了以下方法：

template.html

{% with total=transaction.value*transaction.users.count %}
   R${{ total }}
{% endwith %}

但我收到此错误消息：

/historico/ 处的模板语法错误

无法解析余数：来自“transaction.value*transaction.users.count”的“*transaction.users.count”

Model.py

# An abstract class containing info about a Transaction made by multiple users
class MultipleTransaction (Info):
    # All users that paid the the monthly bill
    users = models.ManyToManyField(User)

    #A file as receipt, it can be an image or a pdf. This is optional
    receipt = models.FileField(
        'Comprovante(s)',
        upload_to='comprovantes',
        blank=True,
        null=True
    )

    class Meta:
        # Make this class an Abstract class
        abstract = True
        # Human friendly singular and plural name
        verbose_name = 'Transação em Grupo'
        verbose_name_plural = 'Transações em Grupo'

View.py

@login_required
def historico(request):
    # Those 3 lines are queries of all transactions
    deposits = chain(Deposit.objects.all(), MonthlyDeposit.objects.all())
    withdraws = chain(Withdraw.objects.all(), EventSubscription.objects.all())
    transferences = Transference.objects.all()

    # Then all transactions are sorted by their atribute "created_at"
    transactions = sorted(
        chain(deposits,withdraws,transferences),
        key=lambda instance: instance.created_at
    )

    # Get all boxes to show their values
    boxes = Box.objects.all()

    return render(
        request,
        'shell/app_shell.html',
        {
            'is_varys': True,
            'transactions': transactions,
            'boxes':boxes,
            'title': 'Histórico'
        }
    )

Answer 1

Django 模板语言不支持乘法。您可以在模型中添加一个方法。

class MultipleTransaction (Info):
    # All users that paid the the monthly bill
    users = models.ManyToManyField(User)

    def calc_total_value(self):
        return self.value * self.users.count()


R${{ transaction.calc_total_value }}

Answer 2

您不能简单地将模板中的变量相乘。您必须编写自己的自定义模板标签以进行乘法运算，或者只是在模型端进行计算。在模型中添加自定义方法来计算总交易价值是一个不错的方法。

Answer 3

试试

transaction.users.all().count

或者

transaction.all()[0].users.count