Validate Django model data by calculating the sum of multiple rows

Question

I have two models:

class User(models.Model):
    name = models.CharField(max_length=32)


class Referral(models.Model):
    referring_user = models.ForeignKey(User, related_name="referrals")
    referred_user  = models.ForeignKey(User, related_name="referrers")
    percentage     = models.PositiveIntegerField()

The idea is that every user has n referrers, and should have at least one. Each referrer has a percentage value which should add up to 100% when added to the other referrers.

So User "Alice" might have referrers "Bob" (50%) and "Cynthia" (50%), and User "Donald" might have one referrer: "Erin" (100%).

The problem I have is with validation. Is there a way (preferably one that plays nice with the Django admin using admin.TabularInline) that I can have validation reject the saving of a User if the sum of Refferrals != 100%?

Ideally I want this to happen at the form/admin level and not by overriding User.save(), but at this point I don't know where to start. Most of Django's validation code appears to be atomic, and validation across multiple rows is not something I've done in Django before.

Answer 1

After Jerry Meng suggested I look into the data property and not cleaned_data, I started poking around admin.ModelAdmin to see how I might access that method. I found get_form which appears to return a form class, so I overrode that method to capture the returning class, subclass it, and override .clean() in there.

Once inside, I looped over self.data, using a regex to find the relevant fields and then literally did the math.

import re
from django import forms
from django.contrib import admin

class UserAdmin(admin.ModelAdmin):

    # ...

    def get_form(self, request, obj=None, **kwargs):

        parent = admin.ModelAdmin.get_form(self, request, obj=None, **kwargs)

        class PercentageSummingForm(parent):
            def clean(self):

                cleaned_data = parent.clean(self)

                total_percentage = 0
                regex = re.compile(r"^referrers-(\d+)-percentage$")

                for k, v in self.data.items():
                    match = re.match(regex, k)
                    if match:
                        try:
                            total_percentage += int(v)
                        except ValueError:
                            raise forms.ValidationError(
                                "Percentage values must be integers"
                            )

                if not total_percentage == 100:
                    raise forms.ValidationError(
                        "Percentage values must add up to 100"
                    )

                return cleaned_data

        return PercentageSummingForm

Answer 2

As per the Django docs, clean() is the official function to implement for your purposes. You could imagine a function that looks like this:

from django.core.exceptions import ValidationError

def clean(self):
    total_percentage = 0
    for referrer in self.referrers.all():
        total_percentage += referrer.percentage
    if total_percentage !== 100:
        raise ValidationError("Referrer percentage does not equal 100")