How to Send List of IDs via multipart/form-data in Django REST Framework

I am working on a Django REST Framework API where I need to send a list of IDs in a multipart/form-data request to create a many-to-many relationship in the database. While everything works perfectly when using JSON as the request format, I face issues when switching to multipart/form-data because the list of IDs doesn't serialize correctly.

Here's my serializer:

class AddOrUpdateContractSerializer(serializers.ModelSerializer):
    deputy_agent = serializers.ListField(
        child=serializers.IntegerField(), required=False, allow_empty=False
    )
    product_type = serializers.ListField(
        child=serializers.IntegerField(), required=False, allow_empty=False
    )
    referral_type = serializers.ListField(
        child=serializers.IntegerField(), required=False, allow_empty=False
    )
    customer_id = serializers.IntegerField(required=False)

    class Meta:
        model = Contract
        exclude = ["id", "product", "is_deleted", "state", "customer"]

Here's my view

class AddContract(APIView):
    @extend_schema(request=AddOrUpdateContractSerializer, tags=["contract"])
    def post(self, request):
        serialized_data = AddOrUpdateContractSerializer(data=request.data)
        if serialized_data.is_valid(raise_exception=True):
            service = ContractService(
                serialized_data=serialized_data.validated_data, user=request.user
            )
            service.create_contract()
            return Response(
                status=status.HTTP_200_OK,
                data={"detail": "contract created successfully"},
            )

And the service for creating the contract:

def create_contract(self):
    items = ["customer_id", "deputy_agent", "product_type", "referral_type"]

    customer = CustomerSelector.get_customer_by_id(
        self.serialized_data.get("customer_id", None)
    )
    deputy_agent = self.serialized_data.get("deputy_agent", None)
    product_type = self.serialized_data.get("product_type", None)
    referral_type = self.serialized_data.get("referral_type", None)

    self.remove_unnecessary_items(items)

    contract = Contract(customer=customer, **self.serialized_data)

    contract.full_clean()
    contract.save()

    if deputy_agent:
        deputy_agents = DeputyAgent.objects.filter(id__in=deputy_agent)
        contract.deputy_agent.add(*deputy_agents)

    if product_type:
        product_types = ComboBoxsOptions.objects.filter(id__in=product_type)
        contract.product_type.add(*product_types)

    if referral_type:
        referral_types = ComboBoxsOptions.objects.filter(id__in=referral_type)
        contract.referral_type.add(*referral_types)

PROBLEM

When sending the data as multipart/form-data, lists like deputy_agent, product_type, and referral_type are received as strings instead of lists

Why I Need multipart/form-data I cannot use JSON because I also need to upload image files along with these IDs in the same request.

Example Request

Content-Type: multipart/form-data

customer_id: 1  
deputy_agent: [1, 2, 3]  
product_type: [4, 5, 6]  
image: <uploaded_file>

Question How can I handle lists of IDs in multipart/form-data properly in Django REST Framework? Is there a best practice or workaround for this issue?

The issue you're encountering is related to how Django handles multipart/form-data and how it serializes data sent in that format. When you send lists like deputy_agent, product_type, and referral_type in a multipart/form-data request, they get serialized as strings, not as actual lists.

To solve this, you can make use of a custom serializer method that handles the parsing of those string representations of lists into actual Python lists.

Solution:

  1. Override the to_internal_value method: This will allow you to manually parse the string into a list.

  2. Use ListField in conjunction with a custom method: Since the data is being sent as a string in multipart/form-data, you need to manually handle this conversion before it gets validated.

Here’s how you can modify your serializer to handle lists sent in multipart/form-data:

Updated Serializer:

from rest_framework import serializers

class AddOrUpdateContractSerializer(serializers.ModelSerializer):
    deputy_agent = serializers.ListField(
        child=serializers.IntegerField(), required=False, allow_empty=False
    )
    product_type = serializers.ListField(
        child=serializers.IntegerField(), required=False, allow_empty=False
    )
    referral_type = serializers.ListField(
        child=serializers.IntegerField(), required=False, allow_empty=False
    )
    customer_id = serializers.IntegerField(required=False)

    class Meta:
        model = Contract
        exclude = ["id", "product", "is_deleted", "state", "customer"]

    def to_internal_value(self, data):
        # If any of the lists are passed as a string, try to parse them into a list
        for field in ['deputy_agent', 'product_type', 'referral_type']:
            if field in data and isinstance(data[field], str):
                # Try to convert string to a list
                try:
                    data[field] = [int(i) for i in data[field].strip('[]').split(',')]
                except ValueError:
                    raise serializers.ValidationError(f"Invalid format for {field}, should be a list of integers.")
        return super().to_internal_value(data)

Key Changes:

  1. to_internal_value method: This method checks if the field is received as a string (which can happen in multipart/form-data), and attempts to parse it into a list of integers.

  2. split(','): Assumes that the list is being sent in a format like '1,2,3'. You can adjust the parsing logic if the format differs.

Example Request (Multipart Form Data):

Here’s what the request should look like when sent as multipart/form-data:

POST /api/contracts/ HTTP/1.1
Content-Type: multipart/form-data; boundary=----WebKitFormBoundaryE19zNvXG3RzrU5J3

------WebKitFormBoundaryE19zNvXG3RzrU5J3
Content-Disposition: form-data; name="customer_id"

1
------WebKitFormBoundaryE19zNvXG3RzrU5J3
Content-Disposition: form-data; name="deputy_agent"

1,2,3
------WebKitFormBoundaryE19zNvXG3RzrU5J3
Content-Disposition: form-data; name="product_type"

4,5,6
------WebKitFormBoundaryE19zNvXG3RzrU5J3
Content-Disposition: form-data; name="image"; filename="image.jpg"
Content-Type: image/jpeg

<image_binary_data>
------WebKitFormBoundaryE19zNvXG3RzrU5J3--

Final Notes:

  • Field Parsing: The to_internal_value method ensures that the deputy_agent, product_type, and referral_type fields are parsed from strings into proper Python lists.
  • File Upload: The image field will still work as expected in multipart/form-data, as Django handles file uploads separately.

This should fix the issue you're facing when working with lists in multipart/form-data in Django REST Framework.

Let me know how this works out!

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