Problem with writing a django rest serializer

Question

I'm trying to write a Django Rest Framework serializer. It is supposed to take in a simple primitive data and return data with some of the related objects data. The model I'm serializing is Model_C

Here are the models:

class ModelA(models.Model):
    name = models.Charfield(max_length=16)
    attr1 = models...
    ...

class ModelB(models.Model):
    name = models.CharField(max_length=16)
    attr1 = models...
    ...

class ModelC(models.Model):
    model_a = models.ForeignKey(ModelA)
    model_b = models.ForeignKey(ModelB)
    text = models.CharField(max_length=32)
    date = models.DateTimeField()

Here is the api view:

class ModelCApiView(RetrieveUpdateDestroyAPIView):
    serializer_class = ModelCSerializer

    def get_queryset(self):
        return self.serializer_class.Meta.model.objects.all()

    def post(self, request):
        serializer = self.get_serializer(data=request.data)
        if serializer.is_valid():
            serializer.save()
            return Response(data=serializer.data, status=status.HTTP_201_CREATED)
        else:
            return Response(data=serializer.errors, status=status.HTTP_400_BAD_REQUEST)

The data fed to the serializer are as follows (with or without the ID):

{'id': '1', 'model_a': 1, 'model_b': 1, 'text': 'some text', 'date': '2019-08-28 08:00:00'}

Now, the thing is that the serializer should save the instance and return the serialized model WITH SOME OF THE related data:

{'id': '1', 'model_a': {'id': 1, 'name': 'some model a name'}, 'model_b': {'id': 1, 'name': 'some model b name', 'attr1': 'some attr value'}, 'text': 'some text', 'date': '2019-08-28 08:00:00'}

So the only thing that is created is ModelC instance. The api view is not supposed to create ModelA or ModelB instances.

The main problem is with serializing/deserializing objects. I already tried several things:

1. The most simple

class ModelCSerializer(serializers.ModelSerializer):
    class Meta:
        model = ModelC
        fields = "__all__"

It properly deserializes the request, saves instance, but returns too little information - only IDs of related objects.

---

2. I used depth

class ModelCSerializer(serializers.ModelSerializer):
    class Meta:
        model = ModelC
        fields = "__all__"
        depth = 1

This does not work, as it spits out 500 with error: "Column 'model_a_id' cannot be null".

---

3. I tried adding to 2. declared serializers:

class ModelCSerializer(serializers.ModelSerializer):
    class Meta:
        model = ModelC
        fields = "__all__"
        depth = 1
    model_a = ModelASerializer()
    model_b = ModelBSerializer()

This produces error 400, bad request, as it asks me to insert ALL data for model_a and model_b as a dict.

---

4. I added lines to APIView to serialize data with simple serializer, and then serialize again with more advanced and return more info.

    serializer = ModelCSerializer

    def post(self, request):
        serializer = self.get_serializer(data=request.data)
        if serializer.is_valid():
            obj = serializer.save()
            serializer = ModelCReadOnlySerializer(instance=obj, data=serializer.data)
            serializer.is_valid()
            return Response(data=serializer.data, status=status.HTTP_201_CREATED)
        else:
            return Response(data=serializer.errors, status=status.HTTP_400_BAD_REQUEST)


class ModelCReadOnlySerializer(serializers.ModelSerializer):
    class Meta:
        model = ModelC
        fields = ("id", "model_a", "model_b", "text", "date")
        read_only_fields = fields
        depth = 1

This is the closest to the solution as I have been, since it returns almost what I need, except for that it returns ALL data for model_a and model_b, which is way too much.

---

At this point I have other ideas, but I bet they're all terrible. I'd be glad on any hints how to proceed, because I'm this close -> || <- to writing everything ny myself without DRF :-)

Thank you in advance.

Answer 1

With great help from @dirkgroten:

Number four is the way to go. First, there have to be two serializers: one for write (as per 1.) and one for read:

class ModelCReadOnlySerializer(serializers.ModelSerializer):
    class Meta:
        model = ModelC
        fields = ("id", "model_a", "model_b", "text", "date")
        read_only_fields = fields
    # Not there is no depth any more
    model_a = ModelASerializer()
    model_b = ModelBSerializer()

And in the view, data does not need to be passed into read-only serializer:

if serializer.is_valid():
            obj = serializer.save()
            serializer = ModelCReadOnlySerializer(instance=obj)
            return Response(data=serializer.data, status=status.HTTP_201_CREATED)
        else:
            return Response(data=serializer.errors, status=status.HTTP_400_BAD_REQUEST)

Now the data is received in simple form, processed and saved, and then more details are returned.

Great thanks once more!

Answer 2

Another way is to override .to_representation(instance) method on the serializer to customize the way output is returned. It takes a little more space, makes little less readable, but makes it more consise and puts all serialization logic into one serializer.