Error when serializing foreign key model images

[Enzodtz]

Hey,

I have a model Pet and another model PetImage, that has a foreign key to Pet. When I'm returning a Pet from the model serializer create method, I think that it might be trying to serialize all model's PetImages, but it is having a problem with their manager, since it raises the exception: 'RelatedManager' object is not iterable.

I'm not sure if the cause of the problem is right, but please take a look on the serializer code, so I can explain it better:

class PetSerializer(serializers.ModelSerializer):
    images = serializers.ListField(child=serializers.ImageField()) 
    # if I comment the above line, and all the images field references below, the error is not raised anymore 

    class Meta:
        model = Pet
        fields = "__all__"
        extra_kwargs = {
            "user": {
                "default": serializers.CurrentUserDefault(),
            },
            "images": {
                "write_only": True,
            },
        }

    def create(self, validated_data):
        images = validated_data.pop("images")
        pet = Pet.objects.create(**validated_data)

        for image in images:
            PetImage.objects.create(image=image, pet=pet)

        # Also, please notice that the error is only raised in the return line, the above lines run perfectly, and the objects are created
        # That's why I think that the error is when serializing.
        return pet

I think that the error is pretty well explained and tracked, but I'm maybe not deep enought into DRF to figure out what's happening after that return statement.

Thanks a lot!

Additional resources:

Models Code

class Pet(models.Model):
    name = models.CharField(max_length=50)
    location = models.CharField(max_length=50)
    user = models.ForeignKey(User, on_delete=models.CASCADE)


class PetImage(models.Model):
    pet = models.ForeignKey(Pet, related_name="images", on_delete=models.CASCADE)
    image = models.ImageField(upload_to="pets")

View Code

class PetView(viewsets.ModelViewSet):
    serializer_class = PetSerializer
    queryset = Pet.objects.all()

[Enzodtz]

I manage to track the problem a bit more into the source code: seems that the serialization of the field is maybe not implemented (or I'm using it wrong haha). But please take a look on what I got:

rest_framework/fields.py: 1670

def to_representation(self, data):
    """
    List of object instances -> List of dicts of primitive datatypes.
    """
    return [self.child.to_representation(item) if item is not None else None for item in data]

As you can see, the function is iterating thought data, but it shouldn't, since for this manager object you have to use the .all() method before iterating on it. If we change these lines to:

    def to_representation(self, data):
        """
        List of object instances -> List of dicts of primitive datatypes.
        """
        related_manager_type = "<class 'django.db.models.fields.related_descriptors.create_reverse_many_to_one_manager.<locals>.RelatedManager'>"
        if str(type(data)) == related_manager_type: # sorry abt the bad type comparison method
            data = data.all()
            print(data)    

        return [self.child.to_representation(item) if item is not None else None for item in data]

The error disappears, and data is printed as a list of PetImage objects. But the images field is not being serialized as it should, since the request output is:

{
	"id": "<pet_id>",
	"images": [
		null
	],
	"name": "<name>",
	"location": "<location>",
	"user": "<user_id>"
}

Maybe this is related to the to_representation method also not have been implemented for the image fields?

Also: I set images field as write_only, but it's still returning it inside the json...

[Enzodtz]

Ended up doing this way:

class PetSerializer(serializers.ModelSerializer):
    images = serializers.SerializerMethodField()

    class Meta:
        model = Pet
        fields = "__all__"
        extra_kwargs = {
            "user": {
                "default": serializers.CurrentUserDefault(),
            },
        }

    def get_images(self, pet):
        images = PetImage.objects.filter(pet=pet)

        urls = []
        for image_obj in images:
            urls.append(image_obj.image.url)

        return urls

    def validate(self, data):
        request = self.context.get("request")

        images = request.FILES.getlist("images")

        if len(images) > 0:
            data["images"] = images
            return data
        else:
            raise serializers.ValidationError({"images": "You must submit an image."})

    def create(self, validated_data):
        images = validated_data.pop("images")

        pet = Pet.objects.create(**validated_data)

        for image in images:
            PetImage.objects.create(image=image, pet=pet)

        return pet

The problem was that I needed to create images field totally from scratch, and that's not ideal. It would be nice if List + Img works, and also, if you could use SerializerMethodField with an write field.

[Smixi]

Well, it does works properly. What you want is to use the "many" arguments and use your own serializer as you did already:

If i'm correct, listfield is more complex than that, and shouldn't be use like that. It's more likely you will use "many" because of how Listfield are created by DRF Fields.

class PetSerializer(serializers.ModelSerializer):
    images = serializers.ListField(child=serializers.ImageField()) 
    # if I comment the above line, and all the images field references below, the error is not raised anymore 

    class Meta:
        model = Pet
        fields = "__all__"
        extra_kwargs = {
            "user": {
                "default": serializers.CurrentUserDefault(),
            },
            "images": {
                "write_only": True,
            },
        }

If you wanted to have nested serializer with create support, either implement it yourself like you did, or use extra like https://github.com/beda-software/drf-writable-nested

You have an example of how to perform that here : https://www.django-rest-framework.org/api-guide/relations/#nested-relationships