Django rest framework does model field validation before own validation - validation

Question

I don't understand django rest framework validation process.
The doc say that validation is performed entirely on the serializer class.
However,
In the case of a PositiveIntegerField in a Model, when ModelSerializer receive a negative value, the model validation is processed before the custom ModelSerializer validate_myfield method.
class Blop(models.Model):
quantity = models.PositiveSmallIntegerField()
class BlopSerializer(serializers.ModelSerializer):
def validate_quantity(self, value):
return max(15, value)
class Meta:
model = Blop
if quantity if lesser than zero, validate_quantity is never called...
I've also tried :
to use model "clean" method but rest framework doesn't use it.
to disable min value validator in model by setting min_value to None, but it make rest framework to crash
to force MinValueValidator in model field : validators=[MinValueValidator(-math.inf, 'Minimum value is -Infinity')], it work, but it's too ugly
How can I do to have a serializer ensure non negative value in this case ?

Answer 1

I think your problem is that you are invoking the serializer.is_valid() method with no kwargs or raise_exception=False.
With raise_exception=True the serializer raises ValidationError, if you pass invalid data.
For better understanding, lets take a look at the Serializer.to_internal_value method.
def to_internal_value(self, data):
"""
Dict of native values <- Dict of primitive datatypes.
"""
if not isinstance(data, dict):
message = self.error_messages['invalid'].format(
datatype=type(data).__name__
)
raise ValidationError({
api_settings.NON_FIELD_ERRORS_KEY: [message]
})
ret = OrderedDict()
errors = OrderedDict()
fields = self._writable_fields
for field in fields:
validate_method = getattr(self, 'validate_' + field.field_name, None)
primitive_value = field.get_value(data)
try:
validated_value = field.run_validation(primitive_value)
if validate_method is not None:
validated_value = validate_method(validated_value)
except ValidationError as exc:
errors[field.field_name] = exc.detail
except DjangoValidationError as exc:
errors[field.field_name] = list(exc.messages)
except SkipField:
pass
else:
set_value(ret, field.source_attrs, validated_value)
if errors:
raise ValidationError(errors)
return ret
As you can see here, firstly, the serializer invokes the field.run_validation method, which uses django-fields validators and they raise DjangoValidationError exception and because of that, your validation method is never invoked.