django rest framework: set field-level error from serializer validate() method

Question

I have a serializer that validates fields based on the values of other fields, In the error response I would like to show each field error as a field error as opposed to showing everything under "non_field_errors" which is what would happen if I were to raise a ValidationError in the object-level validate method. Below is an illustration of what I'm trying to achieve:

MySerializer(ModelSerializer):
    ...
    def validate(self, data):
        field_val1 = data['field_val1']
        field_val2 = data['field_val2']
        if not self._is_field_valid(field_val1, field_val2):
            # The below line is how I would do what I want with Django
            # Forms, however, it's not valid in DRF
            self._errors['field_val1'] = 'this field is not valid'

The desired error response is:

{'field_val1': ['this field is not valid']}

Menno · Accepted Answer · 2016-03-10 14:09:45Z

75

I figured it out, on this page of the documentation in the "BaseSerializer" section, there's an example that shows ValidationError can take a dictionary argument upon initialization.

If I raise ValidationError({'field_val1': ['this field is not valid']}) I get the JSON response I want.

edited Mar 10, 2016 at 14:09

Menno

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answered Jan 27, 2015 at 21:46

Jkk.jonah

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3 Comments

gornvix Over a year ago

Also works well to mimic 'This field is required' responses.

Ralfp Over a year ago

@MarkMishyn no need to, but it's convention set by Django and followed by Rest Framework for fields errors to be lists of errors. But its up to your frontend/client logic to actually decide how to support them.

Amir Savand Over a year ago

Note that by doing this you also have to handle translation yourself, unlike @frnhr's answer.

frnhr · Accepted Answer · 2021-03-17 11:08:48Z

4

Similarly to the answer by @Jkk.jonah, this raises a ValidationError, but it reuses the original exception text without need to re-implement translations:

try:
    serializer.fields['field_val1'].fail('required')
except ValidationError as exc:
    raise ValidationError({
        'field_val1': exc.detail,
    })

By default (i.e. on rest_framework.fields.Field class), available keys are:

default_error_messages = {
    'required': _('This field is required.'),
    'null': _('This field may not be null.')
}

Subclasses can add their own error messages there (and Serializer is a subclass of Field).

BTW, new error messages will be automagically merged with existing (inherited) messages - won't be overridden as might be expected.

edited Mar 17, 2021 at 11:08

answered Jul 6, 2016 at 11:17

frnhr

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1 Comment

Amir Savand Over a year ago

Using the built-in error messages is great for me because the translation is already there.

Erick M · Accepted Answer · 2016-10-03 12:36:57Z

1

If you have logic that applies to all the fields, you can still get the desired result by doing this:

def validate(self, data):
    for key, value in data.items():
        # checks if value is empty
        if not value:
            raise serializers.ValidationError({key: "This field should not be left empty."})

    return data

answered Oct 3, 2016 at 12:36

Erick M

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Comments

Vladyslav · Accepted Answer · 2019-03-10 15:40:39Z

0

In case you are using built-in validators in DRF (which are in-fact django core validators) you have to preprocess django ValidationError coming from validator with a function get_error_detail drf is using for this purpose.

def _validate_min_value(self, data, key):
        try:
            MinValueValidator(Decimal('0.01'))(data.get(key))
        except ValidationErrorDjango as exc:
            raise ValidationError(
                {key: get_error_detail(exc)}
            )

Note that ValidationErrorDjango is a ValidationError from django.core.exceptions, while ValidationError is a one from rest_framework.exceptions

edited Mar 10, 2019 at 15:40

Vladyslav

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answered Feb 12, 2019 at 14:06

okrutny

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