Custom writable field in ModelSerializer

Question

I'd like to add a custom field to a serializer that's used when creating resources. It's not a model field.

I tried the following:

class CampaignSerializer(ModelSerializer):
    class Meta:
        model = Campaign
        fields = ("groups",)
        write_only_fields = ("groups",)

    groups = ListField(IntegerField(), min_length=1)

    def validate(self, data):
        # ...
        return data

However groups doesn't exist in data in the validate() function. I found out that DRF sets read_only=True for the field, which is definitely not what I want.

Is there a way to specify a writable field, or do I have to resort to the view set's perform_create() method?

Answer 1

The following will work, but is likely not the intended way of doing it:

from rest_framework import serializers


class MySerializer(serializers.ModelSerializer):
    class Meta:
        model = MyModel
        fields = ('foo', )  # also add your normal model fields

    foo = serializers.SerializerMethodField()

    def get_foo(self, obj):
        return ...  # do necessary reading stuff

    def create(self, validated_data):
        # use self.initial_data to access the raw input data sent in POST request
        self.initial_data['foo']  
        ...  # do necessary validations of 'foo'
        instance = super().create(validated_data)
        ...  # do necessary write stuff
        return instance

    # Do likewise with .update method