Django Custom Model Field - Positive Decimal Field

Custom Model Field

We saw in the article Custom Model Field Validator - Django, you can use a custom validator for your model fields. However if it's a common field for your models it's better to create a custom field.

In this article we're going to create PositiveDecimalField which only allows positive values.

# src/apps/models.py
from django.db.models import DecimalField
from django.core.exceptions import ValidationError
from django.utils.functional import cached_property
from django.utils.translation import gettext_lazy as _

def validate_positive(value):
    if value < 0:
        raise ValidationError(
            _("%(value)s is not positive."),
            params={"value": value}
        )


class PositiveDecimalField(DecimalField):
    description = _("Positive decimal number")

    @cached_property
    def validators(self):
        return super().validators + [validate_positive]

With PositiveDecimalField we inherit DecimalField class and overwrite it's description attribute and validators method with adding our custom validator.

You can also use Django's built in MinValueValidator like
:super().validators + [MinValueValidator("0.0")].

In adddition to that you can also inherit __init__ method of DecimalField and add your custom validator to validators attribute:

# src/apps/models.py
class PositiveDecimalField(DecimalField):
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.validators.append(validate_positive)

Usage

You can use your custom field as below:

# src/apps/models.py
from django.db import models


class Book(models.Model):
    author = models.ForeignKey(Author, on_delete=models.CASCADE)
    title = models.CharField(max_length=120)
    price = models.PositiveDecimalField()

Unittest

It's very important to write unittest for your code. Here is the test for PositiveDecimalField:

# tests/test_models.py
from django.core.exceptions import ValidationError
from django.test import TestCase

from src.apps.models import PositiveDecimalField


class PositiveDecimalFieldTests(TestCase):
    def test_negative_value(self):
        field = PositiveDecimalField(max_digits=4, decimal_places=2)
        msg = "%s is not positive."
        tests = [
            "-1.3",
            "-0.23",
        ]
        for value in tests:
            with self.subTest(value):
                with self.assertRaisesMessage(ValidationError, msg % (value,)):
                    field.clean(value, None)

You don't need to test DecimalField since Django already has tests for default DecimalField but if you want to make whole test suite:

# tests/test_models.py
from decimal import Decimal

from django.core.exceptions import ValidationError
from django.test import TestCase

from src.apps.models import PositiveDecimalField

class PositiveDecimalFieldTests(TestCase):
    def test_to_python(self):
        f = PositiveDecimalField(max_digits=4, decimal_places=2)
        self.assertEqual(f.to_python(3), Decimal("3"))
        self.assertEqual(f.to_python("3.14"), Decimal("3.14"))
        # to_python() converts floats and honors max_digits.
        self.assertEqual(f.to_python(3.1415926535897), Decimal("3.142"))
        self.assertEqual(f.to_python(2.4), Decimal("2.400"))
        # Uses default rounding of ROUND_HALF_EVEN.
        self.assertEqual(f.to_python(2.0625), Decimal("2.062"))
        self.assertEqual(f.to_python(2.1875), Decimal("2.188"))

    def test_invalid_value(self):
        field = PositiveDecimalField(max_digits=4, decimal_places=2)
        msg = "“%s” value must be a decimal number."
        tests = [
            (),
            [],
            {},
            set(),
            object(),
            complex(),
            "non-numeric string",
            b"non-numeric byte-string",
        ]
        for value in tests:
            with self.subTest(value):
                with self.assertRaisesMessage(ValidationError, msg % (value,)):
                    field.clean(value, None)

    def test_default(self):
        f = PositiveDecimalField(default=Decimal("0.00"))
        self.assertEqual(f.get_default(), Decimal("0.00"))

    def test_get_prep_value(self):
        f = PositiveDecimalField(max_digits=5, decimal_places=1)
        self.assertIsNone(f.get_prep_value(None))
        self.assertEqual(f.get_prep_value("2.4"), Decimal("2.4"))

    def test_negative_value(self):
        field = PositiveDecimalField(max_digits=4, decimal_places=2)
        msg = "%s is not positive."
        tests = [
            "-1.3",
            "-0.23",
        ]
        for value in tests:
            with self.subTest(value):
                with self.assertRaisesMessage(ValidationError, msg % (value,)):
                    field.clean(value, None)