Many to Many field and Nested Serializer in REST: Overwriting Nested serializer doesn't create nested object in Django

Question

I have two models with a many to many relation and I am trying to send nested data to my API. Unfortunately it only gives me back an empty array.

This is what I am trying:

my models:

class Building(models.Model):
    name  = models.CharField(max_length=120, null=True, blank=True)
    net_leased_area = models.FloatField(null=True, blank=True)

class BuildingGroup(models.Model):
    description           = models.CharField(max_length=500, null=True, blank=True)
    buildings             = models.ManyToManyField(Building, default=None, blank=True)

My generic API view:

class BuildingGroupCreateAPIView(CreateAPIView):
    queryset                    = BuildingGroup.objects.all()
    serializer_class            = BuildingGroupSerializer

My serializer:

class BuildingGroupSerializer(serializers.ModelSerializer):

    buildings = BuildingSerializer(many=True)

    class Meta:

        model = BuildingGroup

        fields = (
            'description',
            'buildings',
        )

    def create(self, validated_data):
        buildings_data = validated_data.pop('buildings')
        building_group = BuildingGroup.objects.create(**validated_data)
        for building_data in buildings_data:
            Building.objects.create(building_group=building_group, **building_data)
        return building_group

When I send data it returns this:

{"description":"Test Description API","buildings":[]}

In the array I would like to have my array of dictionaries.

I am trying to follow the REST documentation here when I am overwriting the create method to send a nested object. (https://www.django-rest-framework.org/api-guide/relations/#writable-nested-serializers) and I thought I am doing this correctly, but epic fail.

I send data with request with my custom method like this:

test_api_local(method="post", data={
        "description": "Test Description API",
        "buildings": [{'name' : 'Testname'}, .... ],
         })

Any help is very appreciated. Thanks so much!!

EDIT: When I try to test it on the REST view it tells me:

TypeError: 'building_group' is an invalid keyword argument for this function

EDIT2: Here is my view:

class BuildingGroupCreateAPIView(CreateAPIView):
    queryset                    = BuildingGroup.objects.all()
    serializer_class            = BuildingGroupSerializer

    def create(self, request, *args, **kwargs):
        serializer = BuildingGroupSerializer(data=self.request.data)
        serializer.is_valid(raise_exception=True)
        serializer.save()
        return Response(serializer.data)

Answer 1

You have to explicitly get or create the Building instances, depending upon passed id's inside the payload data, then add them to BuildingGroup instance.

class NestedBuildingSerializer(serializers.ModelSerializer):
    id = serializers.IntegerField(required=False)

    class Meta:
        model = Building
        fields = '__all__'


class BuildingGroupSerializer(serializers.ModelSerializer):
    buildings = NestedBuildingSerializer(many=True)

    class Meta:
        model = BuildingGroup
        fields = (
            'description',
            'buildings',
        )

    def create(self, validated_data):
        buildings_data = validated_data.pop('buildings')
        building_group = BuildingGroup.objects.create(**validated_data)
        buildings = []  # it will contains list of Building model instance
        for building_data in buildings_data:
            building_id = building_data.pop('id', None)
            building, _ = Building.objects.get_or_create(id=building_id,
                                                         defaults=building_data)
            buildings.append(building)
        # add all passed instances of Building model to BuildingGroup instance
        building_group.buildings.add(*buildings)
        return building_group

class BuildingGroupView(ListAPIView, CreateAPIView):
    queryset = BuildingGroup.objects.all()
    serializer_class = BuildingGroupSerializer


## Assume you add your views like this in urls.py
urlpatterns = [
    .....
    path('building-groups', views.BuildingGroupView.as_view(),
         name='building-group'),
    .....
]

On calling endpoint /building-groups as POST method with payload like this:

{
  "description": "here description",
  "buildings": [
    {
      "id": 1, # existing building of id 1
      "name": "name of building 1",
      "net_leased_area": 1800
    },
    { 
      # a new building will gets create
      "name": "name of building 2",
      "net_leased_area": 1800
    }
  ]
}

Then , it will return response like this:-

{
  "description": "here description",
  "buildings": [
    {
      "id": 1,
      "name": "name of building 1",
      "net_leased_area": 1800
    },
    {
      "id": 2
      "name": "name of building 2",
      "net_leased_area": 1800
    }
  ]
}

Learn about M2M relationship and, .get_or_create()

Note: BuildingSerializer and NestedBuildingSerializer are both different. Don't mix them up.