Django Rest Framework validator not execute inside extra_kwargs

Question

I have Administrator module extended from default Django User module.

In Django Rest Framework, I create a serializer for this module with username and email validators.

Everything go well when I declare validators inlined:

class AdministratorCreateUpdateSerializer(ModelSerializer):

    username = serializers.CharField(
        source='user.username',
        validators=[UniqueValidator(queryset=User.objects.all())]
    )
    email = serializers.EmailField(
        source='user.email',
        validators=[UniqueValidator(queryset=User.objects.all())]
    )
    password = serializers.CharField(
        source='user.password',
        allow_blank=True,
        style={'input_type': 'password'}
    )
    first_name = serializers.CharField(
        source='user.first_name'
    )
    last_name = serializers.CharField(
        source='user.last_name'
    )

    class Meta:
        model = Administrator
        fields = [
            'username',
            'email',
            'password',
            'first_name',
            'last_name',
        ]

But the validators not execute when I declare it inside extra_kwargs:

class AdministratorCreateUpdateSerializer(ModelSerializer):

    username = serializers.CharField(
        source='user.username',
    )
    email = serializers.EmailField(
        source='user.email',
    )
    password = serializers.CharField(
        source='user.password',
        allow_blank=True,
        style={'input_type': 'password'}
    )
    first_name = serializers.CharField(
        source='user.first_name'
    )
    last_name = serializers.CharField(
        source='user.last_name'
    )

    class Meta:
        model = Administrator
        fields = [
            'username',
            'email',
            'password',
            'first_name',
            'last_name',
        ]
        extra_kwargs = {
            'username': {
                'validators': [UniqueValidator(queryset=User.objects.all())]
            },
            'email': {
                'validators': [UniqueValidator(queryset=User.objects.all())]
            },
        }

Does this problem come from using source when define addition fields or something else?

Answer 1

extra_kwargs do not work for fields which are explicitly declared on the serializer. It's the same case with read_only_fields, which are then passed to extra_kwargs. Each field that's declared explicitly is simply omitted when extra_kwargs are added to the rest of the constructor arguments.

This should be clearly pointed out in the docs but it's not.

discussion: https://github.com/encode/django-rest-framework/issues/3460

code: https://github.com/encode/django-rest-framework/blob/master/rest_framework/serializers.py#L1007

so your first solution is the only proper solution :)