[Django]-DRF serializer check undefined field

You can update the validation method like this:

class BoardSerializer(serializers.Serializer):
    user_id = serializers.IntegerField(required=True)
    body = serializers.CharField(required=False)

    def validate(self, attrs):
        unknown =  set(self.initial_data) - set(self.fields)
        if unknown:
            raise serializers.ValidationError("Unknown field(s): {}".format(", ".join(unknown)))
        return attrs

Here, it will check if there is any extra fields passing through the serializer and compare with existing fields. If it exists, then it will throw error(or make is_valid() == false). For more information, you can check this so answer.