get_related_resource_type failing for polymorphic serializers

[favll]

We have run into a problem where DRF JSON API resolves the wrong related resource type when trying to include a resource using a polymorphic serializer. Specifically, DRF JSON API will resolve the base model instead of the inherited model.

Below is an abbreviated example for the issue we are running into. In the following example utils.get_related_resource_type will resolve the parent_model to models.Product instead of models.ArtProduct (line here). Subsequently, the method will try to access art_details on the parent_model which will fail since it only exists on models.ArtProduct and not models.Product (line here).

A proposed fix would be to resolve the parent_model by checking if the parent_serializer is a polymorphic serializer and resolving the parent_model with something along the lines of:
parent_serializer.get_polymorphic_serializer_for_instance(parent_serializer.instance).Meta.model

If desired, I can provide further clarification and prepare a PR.

class ArtProductSerializer(serializers.ModelSerializer):
    price = MoneyField(max_digits=19, decimal_places=4)

    included_serializers = {
        "art_details": ArtDetailSerializer,
    }

    class JSONAPIMeta:
        included_resources = ("art_details",)
        resource_name = "ArtProduct"

    class Meta:
        fields = (
            "id",
            "price",
            "art_details",
        )
        model = models.ArtProduct


class PolymorphicProductSerializer(serializers.PolymorphicModelSerializer):
    polymorphic_serializers = [ArtProductSerializer]

    class Meta:
        model = models.Product


class CartItemSerializer(serializers.ModelSerializer):

    included_serializers = {
        "product": PolymorphicProductSerializer,
    }

    class Meta:
        model = models.CartItem
        fields = ("id", "product")


class CartSerializer(serializers.ModelSerializer):
    included_serializers = {
        "items": CartItemSerializer,
    }

    class Meta:
        model = models.Cart
        fields = ("id", "items")
        read_only_fields = ("id", "items")

    class JSONAPIMeta:
        included_resources = ("items", "items.product")

[sliverc]

It would be great if you could open a PR which reproduces this error and a potential fix.

Once a reproducing test is ready it might also be clearer whether your suggested fix works or whether there are better solutions.

[ro70]

@favll Probably I ran into the same problem. Any news on this?

[David-Guillot]

@favll @ro70 did you make sure you're properly using the PolymorphicResourceRelatedField as indicated in the docs?

Defining a polymorphic serializer is enough when using it directly for its model endpoint, but for related models, as in your example @favll , a PolymorphicResourceRelatedField referring to your polymorphic serializer should do the trick, i'm seeing it working right now on my own project and you can see it implemented in the example app.

[sliverc]

As there hasn't been any feedback and as of comment of @David-Guillot I assume this issue has been solved so closing. Please don't hesitate to comment if this issue still exists though.