How use count() in a many to many field

Question

My django project it's about money management, so one of it's type of money transaction is MultipleTransaction, it only says that it was made by multiple users together.

I want to show in my interface the total value from one MultipleTransaction, using the Django template engine, how do I do that?

I tried the following:

template.html

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

But I get this error message:

TemplateSyntaxError at /historico/

Could not parse the remainder: '*transaction.users.count' from 'transaction.value*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'
        }
    )

score 3 · Accepted Answer · answered Apr 17 '18 at 18:26

The Django template language does not support multiplication. You could add a method to your model.

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 }}

Olzhas Arystanov · Answer 2 · 2018-04-17T18:28:48.527

1

You can't simply multiply variables in templates. You must either write your own custom template tag for multiplication or simply do calculations on the model side. Adding a custom method in your model to calculate total transaction value is a good way to go.

edited Apr 17 '18 at 18:28

answered Apr 17 '18 at 18:17

Olzhas Arystanov

946
5
8

The error persists : Could not parse the remainder: '*transaction.users.all.count' from 'transaction.value\*transaction.users.all.count' – Kanagawa Marcos Apr 17 '18 at 18:18
Error: Could not parse the remainder: '*transaction.user_set.count' from 'transaction.value\*transaction.user_set.count' – Kanagawa Marcos Apr 17 '18 at 18:22
Got it now. Updated the answer. – Olzhas Arystanov Apr 17 '18 at 18:29
Thank, you, both answer are good. Would be wonderful if you added a template tag example that solve the problem as well. – Kanagawa Marcos Apr 17 '18 at 18:37
Check this [post](https://stackoverflow.com/questions/19588160/multiply-in-django-template/19588661) – Olzhas Arystanov Apr 17 '18 at 18:42

score 0 · Answer 3 · answered Apr 17 '18 at 18:24

0

Try with

transaction.users.all().count

Or

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

answered Apr 17 '18 at 18:24

Benjamin RD

11,516
14
87
157

**error 1:** Could not parse the remainder: '*transaction.users.all().count' from 'transaction.value\*transaction.users.all().count' **error 2:** Could not parse the remainder: '*transaction.all()[0].users.count' from 'transaction.value\*transaction.all()[0].users.count' – Kanagawa Marcos Apr 17 '18 at 18:29

How use count() in a many to many field

3 Answers3