Polymorphic serializer was not found for missing class discriminator (‘null’)

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The error message “polymorphic serializer was not found for missing class discriminator (‘null’)” is usually encountered when using a serializer that expects a discriminating value to be present in the serialized data, but it is missing or not properly specified.

A polymorphic serializer is a serializer that can handle multiple different classes or types in a serialized format. It relies on a class discriminator, which is a value or key that identifies the specific class or type when deserializing the data.

In the context of your error message, it seems that the class discriminator value is either missing or set to ‘null’. This means that the serializer is unable to determine the appropriate class or type to deserialize the data into.

To resolve this issue, you need to ensure that the serialized data includes a valid class discriminator value. The specific steps to do this will depend on the serializer library or framework you are using. Here is a general example using the Django Rest Framework (DRF) in Python: 

from rest_framework import serializers

class MyBaseClassSerializer(serializers.ModelSerializer):
    class Meta:
        model = MyBaseClass
        fields = '__all__'
        # Define the class discriminator field
        discriminator = serializers.CharField()

class MySubClassSerializerOne(MyBaseClassSerializer):
    class Meta:
        model = MySubClassOne

class MySubClassSerializerTwo(MyBaseClassSerializer):
    class Meta:
        model = MySubClassTwo

# Example usage
serialized_data = {
    'field1': 'value1',
    'field2': 'value2',
    'discriminator': 'MySubClassOne'  # Set the class discriminator value
}

serializer = MyBaseClassSerializer(data=serialized_data)
if serializer.is_valid():
    deserialized_object = serializer.save()
else:
    print(serializer.errors)

In the example above, we define a base serializer class MyBaseClassSerializer that includes a discriminator field. This field is used to specify the class or type when deserializing the data. We then define two sub-class serializers MySubClassSerializerOne and MySubClassSerializerTwo that inherit from the base class. Each subclass serializer represents a different class or type in the serialized data.

When serializing or deserializing data, we need to ensure that the class discriminator field is set with the appropriate value. In the example usage, we set the discriminator field to 'MySubClassOne' to indicate that the data represents an instance of MySubClassOne.

By properly setting the class discriminator field in your serialized data, you should be able to resolve the “polymorphic serializer was not found for missing class discriminator (‘null’)” error.

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