djang模型的多态序列化

Question

我有这样的继承模型

基类

class PrivateMessage(Base):
    sender = models.ForeignKey(User, related_name='privatemessage_set', on_delete=models.CASCADE, null=True)
    room = models.ForeignKey(PrivateRoom, on_delete=models.CASCADE, related_name='message_set', null=True)
    message_type = models.CharField(max_length=255, null=True)

孩子们

class PrivateTextMessage(PrivateMessage):
        body = models.TextField(null=True, blank=True)

class PrivateImageMessage(PrivateMessage):
        image = models.ImageField(upload_to=attachment_images_path, null=True, blank=True)

我在用多态的方式序列化它们

class PublicMessageSerializer(serializers.ModelSerializer):
    class Meta:
        model = PublicMessage
        fields = '__all__'

    def to_representation(self, obj):
        if isinstance(obj, PublicTextMessage):
            return PublicTextMessageSerializer(obj, context=self.context).to_representation(obj)
        elif isinstance(obj, PublicImageMessage):
            return PublicImageMessageSerializer(obj, 
        return super(PublicMessageSerializer, self).to_representation(obj)


class PublicTextMessageSerializer(serializers.ModelSerializer):
    sender = UserSerializer(read_only=True)
    receiver = RoomSerializer(read_only=True)

    class Meta:
        model = PublicTextMessage
        fields = ('id', 'sender', 'receiver', 'body', 'message_type', 'created', 'modified',)


class PublicImageMessageSerializer(serializers.ModelSerializer):
    sender = UserSerializer(read_only=True)
    receiver = RoomSerializer(read_only=True)
    image = serializers.ImageField(use_url=True)

    class Meta:
        model = PublicImageMessage
        fields = ('id', 'sender', 'receiver', 'image', 'message_type', 'created', 'modified',)

问题是序列化程序只呈现基类中的字段。例如，PublicImageMessage中的图像字段永远不会呈现

有人能帮助我条件呈现类的所有字段

PS: The classes have a PublicMessageType object. I've tried using that and serializing, but outside of that being clunky, in that case the fields are rendered with null value.