data = { 'a_ref':["17629","17630","17631"] }
x= Model_Serializer(data=data)

class A_Serializer(serializers.ModelSerializer):
    class Meta:
        model = A
        fields = ('pk',)

class Model_Serializer(serializers.ModelSerializer):
    a_ref = A_Serializer( many=True)


def create(self, validated_data):
    tracks_data = validated_data.pop('a_ref')
    model = Model.objects.create(**validated_data)

    for track_data in tracks_data:
        A.objects.create(ref=model, **track_data)
    return model

class Meta:
    model = models.Model

[14]: x.is_valid()
Out[14]: False


x.errors
Out[16]:
ReturnDict([
            ('a_ref',
             [{'non_field_errors': ['Invalid data. Expected a dictionary, but got str.']},
              {'non_field_errors': ['Invalid data. Expected a dictionary, but got str.']},
              {'non_field_errors': ['Invalid data. Expected a dictionary, but got str.']}])])