db_constraint_example.py

class Foo(models.Model):


    def object_validation(
        cls, some_values
    ) -> Optional[str]:
        """
        Validation logic when creating a Foo record

        If creation is via a model serializer's is_valid method this function
        will get triggered via the clean method.

        If creation is not via a model serializer's is_valid method this
        function will get triggered via the save method.
        """
        if not some_value == "somethong":
            return FOO_ERROR_MSG_SOMETHING_GOES_HERE
        return None

    def clean(self):
        """
        `is_valid` calls on model serializers should trigger this validation prior to saving
        """
        error_msg = self.content_object_validation(
            some_value
        )
        if error_msg:
            raise ValidationError(error_msg)

    def save(self, *args, **kwargs):
        """
        Ensure new objects are valid prior to save.
        """
        if not self.id:
            error_msg = self.content_object_validation(
                some_value
            )
            if error_msg:
                raise IntegrityError(error_msg)
        super().save(*args, **kwargs)