Django has it's built-in validators which you can use, however if you want to write your own custom validation:
# models.py
from django.core.exceptions import ValidationError
from django.db import models
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 Book(models.Model):
author = models.ForeignKey(Author, on_delete=models.CASCADE)
title = models.CharField(max_length=120)
price = models.DecimalField(validators=[validate_positive])
With validate_positive validator we ensure that price is always positive.
NOTE
validators param always must be a list, even though there is one validator.
By the way you don't need to use gettext_lazy but it's a good habit for i18n.
All done!
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