Calculated fields example

models.py

from django.db import models
from django.db.models.signals import post_init
from django.db.models.fields import NOT_PROVIDED
from django.core import validators as v


class Substance(models.Model):
    name = models.CharField(max_length=32, primary_key=True)


class MixtureIngredient(models.Model):
    substance = models.ForeignKey(
        Substance, on_delete=models.CASCADE
    )
    mixture = models.ForeignKey(
        'Mixture', on_delete=models.CASCADE
    )
    proportion = models.PositiveSmallIntegerField(
        validators=[
            v.MinValueValidator(1),
            v.MaxValueValidator(100),
        ]
    )


class CalculatedPercentageField(models.PositiveSmallIntegerField):
    pct_model = None
    pct_field_name = None

    def __init__(self, pct_model: models.Model, pct_field_name='percentage',
                 *args, **kwargs):
        self.pct_model = pct_model
        self.pct_field_name = pct_field_name
        super().__init__(*args, **kwargs)

    def deconstruct(self):
        name, path, args, kwargs = super(
            CalculatedPercentageField, self
        ).deconstruct()
        kwargs['pct_model'] = self.pct_model
        kwargs['pct_field_name'] = self.pct_field_name
        return name, path, args, kwargs

    def update_total(self, instance, **kwargs):
        manager = self.pct_model._meta.default_manager
        total = 0
        for value_dict in manager.values(self.pct_field_name):
            total += value_dict[self.pct_field_name]
        setattr(instance, self.name, total)

    def contribute_to_class(self, cls, name, private_only=False,
                            virtual_only=NOT_PROVIDED):
        super(CalculatedPercentageField, self).contribute_to_class(
            cls, name, private_only, virtual_only
        )
        # noinspection PyProtectedMember
        if not cls._meta.abstract:
            post_init.connect(self.update_total, sender=cls)


class Mixture(models.Model):
    name = models.CharField(max_length=63, unique=True)
    ingredients = models.ManyToManyField(Substance, through=MixtureIngredient)
    total_proportion = CalculatedPercentageField(
        MixtureIngredient, 'proportion'
    )

tests.py

from django.test import TestCase
from .models import Substance, Mixture, MixtureIngredient


class MixtureTest(TestCase):
    @classmethod
    def setUpTestData(cls):
        cls.water = Substance.objects.create(name='Water')
        cls.sugar = Substance.objects.create(name='Sugar')
        cls.earl_grey = Substance.objects.create(name='Earl Grey')

    def test_mixture(self):
        tea = Mixture.objects.create(name='tea', total_proportion=0)
        MixtureIngredient.objects.create(
            substance=self.water, mixture=tea, proportion=80
        )
        MixtureIngredient.objects.create(
            substance=self.earl_grey, mixture=tea, proportion=5
        )
        actual = Mixture.objects.get(name='tea')
        self.assertEqual(actual.total_proportion, 85)
        MixtureIngredient.objects.create(
            substance=self.earl_grey, mixture=tea, proportion=15
        )
        actual2 = Mixture.objects.get(name='tea')
        self.assertEqual(actual2.total_proportion, 100)