Django EmbeddedModelField saying "This field may not be blank" when doing PUT request despite field having "blank=True"

Question

I'm creating a Django application with django-rest-framework and using djongo to connect to MongoDB. I have nested models as such:

class Group(models.Model):
    users = models.ArrayModelField(
        model_container=User
    )

class User(models.Model):
    number = models.IntegerField(
        default=None,
        null=True
    )

    song = models.EmbeddedModelField(
        model_container=Song,
        null=True,
        blank=True
    )

    class Meta:
        abstract = True

class Song(models.Model):
    mp3_file = models.URLField(
        default=None,
        blank=True,
        null=True
    )

    comments = models.CharField(
        max_length=250,
        default='',
        blank=True
    )

    class Meta:
        abstract = True

When a Group is created, the Users and the Songs are created without any problems. For example, when created, the Group may look like:

{
    "name": "Sample",
    "users: [
        {
            "number": null,
            "song": {
                "mp3_file": null,
                "comments": ""
            }
        }
    ]
}

This all works fine. However, if I try to do a PUT request and don't change the value of "number", "mp3_file", or "comments", I will get the following error message:

"user": [
    {
        "number": [
            "This field may not be null."
        ],
        "song": {
            "mp3_file": [
                "This field may not be null."
            ],
            "comments": [
                "This field may not be blank."
            ]
        }
    }
]

Any thoughts on how to fix this error? I am just using a generic RetrieveUpdateDestroyAPIView as the view for this endpoint.

edit: I have also tried recreating all migrations as well as dropping the table and recreating it, and neither of those approaches helped.

edit: Here is the view:

class GroupDetail(generics.RetrieveUpdateDestroyAPIView):
    serializer_class = GroupSerializer
    queryset = Group.objects.all()
    lookup_field = 'group_name'
    lookup_url_kwarg = 'group_name'

And the serializers:

class GroupSerializer(serializers.HyperlinkedModelSerializer):
    users = UserSerializer(many=True)

    def update(self, instance, validated_data):
        if validated_data.get('group_name', None) is not None:
            instance.__setattr__('group_name', validated_data.get('group_name'))

        instance.save()

        return instance

    class Meta:
        model = Group
        fields = (
            'group_name',
            'users'
        )


class UserSerializer(serializers.Serializer):
    number = serializers.IntegerField()
    song = SongSerializer()


class SongSerializer(serializers.Serializer):
    mp3_file = serializers.URLField()
    comments = serializers.CharField(
        max_length=250
    )

But part of the issue is that the serializers are never even being reached since the data is validating incorrectly.

Answer 1

try with change:

class UserSerializer(serializers.Serializer):
    number = serializers.IntegerField(allow_null=True)
    song = SongSerializer()

class SongSerializer(serializers.Serializer):
    mp3_file = serializers.URLField(allow_blank=True)
    comments = serializers.CharField(
        max_length=250, allow_blank=True
    )