Symmetric django model

Question

I want to create a model, which is symmetric on two fields. Let's call the model Balance:

class Balance (models.Model):
    payer = models.ForeignKey(auth.User, ...)
    payee = models.ForeignKey(auth.User, ...)
    amount = models.DecimalField(...)

It should have the following property:

balance_forward = Balance.objects.get(payer=USER_1, payee=USER_2)
balance_backward = Balance.objects.get(payer=USER_2, payee=USER_1)

balance_forward.amount == -1 * balance_backward.amount

What is the best way to implement this?

Answer 1

So, I came up with the following solution. Feel free to suggest other solutions.

class SymmetricPlayersQuerySet (models.query.QuerySet):

    def do_swap(self, obj):
        obj.payer, obj.payee = obj.payee, obj.payer
        obj.amount *= -1

    def get(self, **kwargs):
        swap = False
        if "payer" in kwargs and "payee" in kwargs:
            if kwargs["payer"].id > kwargs["payee"].id:
                swap = True
                kwargs["payer"], kwargs["payee"] = \
                    kwargs["payee"], kwargs["payer"]

        obj = super().get(**kwargs)

        if swap:
            self.do_swap(obj)

        return obj

    def filter(self, *args, **kwargs):
        if (
            ("payer" in kwargs and "payee" not in kwargs) or
            ("payee" in kwargs and "payer" not in kwargs)
        ):
            if "payee" in kwargs:
                key, other = "payee", "payer"
            else:
                key, other = "payer", "payee"

            constraints = (
                models.Q(payer=kwargs[key]) |
                models.Q(payee=kwargs[key])
            )

            queryset = super().filter(constraints)
            for obj in queryset:
                if getattr(obj, other) == kwargs[key]:
                    self.do_swap(obj)

            return queryset

        return super().filter(*args, **kwargs)


class BalanceManager (models.Manager.from_queryset(SymmetricPlayersQuerySet)):
    pass


class Balance (models.Model):

    objects = BalanceManager()

    payer = models.ForeignKey(
        Player,
        on_delete=models.CASCADE,
        related_name='balance_payer',
    )
    payee = models.ForeignKey(
        Player,
        on_delete=models.CASCADE,
        related_name='balance_payee',
    )
    amount = models.DecimalField(decimal_places=2, max_digits=1000, default=0)

    def do_swap(self):
        self.payer, self.payee = self.payee, self.payer
        self.amount *= -1

    def save(self, *args, **kwargs):

        swap = False
        if self.payer.id > self.payee.id:
            swap = True
            self.do_swap()

        result = super().save(*args, **kwargs)

        if swap:
            self.do_swap()

        return result

    def refresh_from_db(self, *args, **kwargs):
        swap = False
        if self.payer.id > self.payee.id:
            swap = True

        super().refresh_from_db(*args, **kwargs)

        if swap:
            self.do_swap()

Answer 2

You can aggregate on the Balance objects with:

from django.db.models import Case, F, Sum, When
from django.conf import settings


class Balance(models.Model):
    payer = models.ForeignKey(settings.AUTH_USER_MODEL, …)
    payee = models.ForeignKey(settings.AUTH_USER_MODEL, …)
    amount = models.DecimalField(…)

    def get_balance(cls, payer, payee):
        return cls.objects.filter(
            Q(payer=payer, payee=payee) | Q(payer=payee, payee=payer)
        ).aggregate(
            total=Sum(
                Case(
                    When(payer=payer, then=F('amount')),
                    otherwise=-F('amount'),
                    output_field=DecimalField(…),
                )
            )
        )['total']

This will look for all Balances between the payer and the payee, and subtract the ones in the opposite direction. The Balance.get_balance(payer=foo, payee=bar) will thus determine the total flow from foo to bar.

---

Note: It is normally better to make use of the settings.AUTH_USER_MODEL ^[Django-doc] to refer to the user model, than to use the User model ^[Django-doc] directly. For more information you can see the referencing the User model section of the documentation.