[Fixed]-DRF: Validate nested serializer data when creating, but not when updating


This is how I did it:

class GenreSerializer(serializers.ModelSerializer):
    # ... snip ...
    def validate_name(self, value):
       if self.context['request']._request.method == 'POST':
           if self.Meta.model.objects.filter(name=value).exists():
               raise ValidationError('A genre with this name already exists.')
        return value

In this way the validation is triggered only when a new Genre object is created (POST), not when it is updated (PUT).
When a new Book object is created, the validation for Genre is propagated to the nested serializer.
All form inputs are preserved after validation and the error message is attached to the field name.

That actually fulfills all my criteria. Although I don’t have the feeling that this is the right way of doing it. I’d still like to know how could I call manually the UniqueValidator in validate_name, instead of reinventing that validation.


I found out a way how to call the UniqueValidator in the method:

def validate_name(self, value):
    if self.context['request']._request.method == 'POST':
        unique = UniqueValidator(
            message='Genre with this name already exists.'
    return value

EDIT (2022-09-09):

Instead of self.context['request']._request.method it’s much simpler and cleaner to use self.context['request'].method.

Tried and tested with Django REST Framework 3.13.



What I really miss in both solutions is that the validation error is not attached to the field name of the model Genre and the user input in the form is lost.

This is straight forward:

from rest_framework import exceptions

class BookViewSet(viewsets.ModelViewSet):
    def perform_create(self, serializer):
        if check_failed():
            raise exceptions.ValidationError(
                   'genre': {
                        'name': ['must be unique']


In addition to @cezar’s answer I was wondering how to not just override based on a condition, but to completely override and ignore any of the fields.

It might be possible to "skip" children serializer validation, or write custom serializer validation to override child serializers, by overriding to_internal_value of the ParentSerialzer:

class ParentSerializer(serializers.ModelSerializer):
    children = ChildSerializer(many=True)

    def to_internal_value(self, *args, **kwargs):
            # runs the child serializers
            return super().to_internal_value(*args, **kwargs)
        except ValidationError as e:
            # fails, and then overrides the child errors with the parent error
            return self.validate(self.initial_data)

    def validate(self, attrs):
        errors = {}
        errors['custom_override_error'] = 'this ignores and overrides the children serializer errors'
        if len(errors):
            raise ValidationError(errors)
        return attrs
    class Meta:
        model = Parent

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