Django Rest Framework POST Update if existing or create

Question

I am new to DRF. I read the API docs, maybe it is obvious but I couldn't find a handy way to do it.

I have an Answer object which has one-to-one relationship with a Question.

On the frontend I used to use POST method to create an answer sent to api/answers, and PUT method to update sent to e.g. api/answers/24

But I want to handle it on the server side. I will only send a POST method to api/answers and DRF will check based on answer_id or question_id (since it is one to one) if the object exists. If it does, it will update the existing one, and if it doesn't, it will create a new answer.

I couldn't figure out where I should implement it. Should I override create() in serializer or in ViewSet or something else?

Here are my model, serializer, and view:

class Answer(models.Model):     question = models.OneToOneField(         Question, on_delete=models.CASCADE, related_name="answer"     )     answer = models.CharField(         max_length=1, choices=ANSWER_CHOICES, null=True, blank=True     )   class AnswerSerializer(serializers.ModelSerializer):     question = serializers.PrimaryKeyRelatedField(         many=False, queryset=Question.objects.all()     )      class Meta:         model = Answer         fields = ("id", "answer", "question")   class AnswerViewSet(ModelViewSet):     queryset = Answer.objects.all()     serializer_class = AnswerSerializer     filter_fields = ("question", "answer") 

Answer 1

Unfortunately your provided and accepted answer does not answer your original question, since it does not update the model. This however is easily achieved by another convenience method: update-or-create

def create(self, validated_data):     answer, created = Answer.objects.update_or_create(         question=validated_data.get('question', None),         defaults={'answer': validated_data.get('answer', None)})     return answer 

This should create an Answer object in the database if one with question=validated_data['question'] does not exist with the answer taken from validated_data['answer']. If it already exists, django will set its answer attribute to validated_data['answer'].

As noted by the answer of Nirri, this function should reside inside the serializer. If you use the generic ListCreateView it will call the create function once a post request is sent and generate the corresponding response.

Answer 2

Answer posted by @Nirri helped me as well, but I've found a more elegant solution using Django's QuerySet API shortcut:

def create(self, validated_data):     answer, created = Answer.objects.get_or_create(         question=validated_data.get('question', None),         defaults={'answer': validated_data.get('answer', None)})      return answer 

It does exactly the same thing - if the Answer to that Question does not exist, it will be created, otherwise it is returned as-is by the question field lookup.

This shortcut, however, won't update the object. QuerySet API has another method for an update operation, which is called update_or_create and posted in other answer down the thread.