How to create multiple objects in serializers create method?

Question

I am trying to upload a csv file and then using it to populate a table in the database (creating multiple objects).

serializers.py:

def instantiate_batch_objects(data_list, user):
    return [
        WorkData(
            work=db_obj['work'],
            recordTime=db_obj['recordTime'],
            user=user
        ) for db_obj in data_list
    ]


class FileUploadSerializer(serializers.ModelSerializer):
    filedata = serializers.FileField(write_only=True)

    class Meta:
        model = WorkData
        fields = ['user', 'filedata']

    def create(self, validated_data):
        file = validated_data.pop('filedata')
        data_list = csv_file_parser(file)        
        batch = instantiate_batch_objects(data_list, validated_data['user'])
        work_data_objects = WorkData.objects.bulk_create(batch)
        # print(work_data_objects[0])
        return work_data_objects

views.py:

class FileUploadView(generics.CreateAPIView):
    queryset = WorkData.objects.all()
    permission_classes = [IsAuthenticated]
    serializer_class = FileUploadSerializer

    # I guess, this is not need for my case.
    def get_serializer(self, *args, **kwargs):
        print(kwargs.get('data'))
        if isinstance(kwargs.get('data', {}), list):
            kwargs['many'] = True

        return super().get_serializer(*args, **kwargs)

models.py

class WorkData(models.Model):
    user = models.ForeignKey(
        settings.AUTH_USER_MODEL,
        on_delete=models.CASCADE,
        related_name='work_data',
    )
    work = models.IntegerField(blank=False, null=False)
    recordTime = models.DateTimeField(blank=False, null=True)

When I upload the file and post it I get this error:

Got AttributeError when attempting to get a value for field user on serializer FileUploadSerializer. The serializer field might be named incorrectly and not match any attribute or key on the list instance. Original exception text was: 'list' object has no attribute 'user'.

But I can see table is populated successfully in the database. What should I return from create method of FileUploadSerializer?

FileUploadSerializer.create() supposed to be retrun only a single model instance. — JPG
– JPG, Commented Oct 31, 2020 at 3:21
can you add a snippet to the data you are passing to your endpoint? — Mahmoud Adel
– Mahmoud Adel, Commented Oct 31, 2020 at 9:22
I am passing a file to the api endpoint. Then reading data from the file and creating objects overriding serializer class create method. — haccks
– haccks, Commented Oct 31, 2020 at 9:32

Mahmoud Adel · Accepted Answer · 2020-10-31 14:06:35Z

1

OK, after trying an example myself I was able to reproduce the errors, I have a better understanding of why this is happing now.

First, let's put the implementation of create() on the view class here

def create(self, request, *args, **kwargs):
        serializer = self.get_serializer(data=request.data)
        serializer.is_valid(raise_exception=True)
        self.perform_create(serializer)
        headers = self.get_success_headers(serializer.data)
        return Response(serializer.data, status=status.HTTP_201_CREATED, headers=headers)

The original error of Got AttributeError when attempting to get a value for field user... etc happened because the create() in the FileUploadView is returning serializer.data which is expecting fields user and filedata but create() on FileUploadSerializer is returning a list of objects so you can see now why this is happening.

You can solve this by overriding create() on FileUploadView and serialize the returned serializer data with a WorkDataSerializer that you will create

For ex:

def create(self, request, *args, **kwargs):
        serializer = self.get_serializer(data=request.data)
        serializer.is_valid(raise_exception=True)
        self.perform_create(serializer)
        headers = self.get_success_headers(serializer.data)
        workData = WorkDataSerializer(data=serializer.data)
        return Response(workData.data, status=status.HTTP_201_CREATED, headers=headers)

OR, you can do it on serializer level - which I prefer -

For example:

class FileUploadSerializer(serializers.ModelSerializer):
    filedata = serializers.FileField(write_only=True)
    created_objects_from_file = serializers.SerializerMethodField()
        
    def get_created_objects_from_file(self, obj):
       file = self.validated_data.pop('filedata')
       data_list = csv_file_parser(file)        
       batch = instantiate_batch_objects(data_list, self.validated_data['user'])
       work_data_objects = WorkData.objects.bulk_create(batch)
       
       return WorkDataSerializer(work_data_objects, many = True).data
    
    
    class Meta:
        model = WorkData
        fields = ['user', 'filedata']



class WorkDataSerializer(serializers.Serializer):
         # fields of WorkData model you want to return

This should work with no problems, note that SerializerMethodField is read_only by default

see https://www.django-rest-framework.org/api-guide/fields/#serializermethodfield

edited Oct 31, 2020 at 14:06

answered Oct 31, 2020 at 12:08

Mahmoud Adel

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9 Comments

haccks Over a year ago

My upvote for finding the problem. I too figured it out and the naive solution came to my mind is to just return one object from list: return work_data_objects[0].

Mahmoud Adel Over a year ago

I assumed that returning all the created objects is mandatory or something, glad that you figured it out.

Mahmoud Adel Over a year ago

I will update my previouse answer so nobody get confused later

haccks Over a year ago

No, it wasn't. But looking for a clean solution right now. It would be great if you can put solution using ListSerialzer.

haccks Over a year ago

I would suggest please test your solution and post it only after it works. I can see lots of errors in second solution.

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