Append an object to a list with django GenericViewSet

Question

I wrote a many-to-many field for my model but when I try to write an updated view with generic-view-set don't append the new obj just replace it,how can I write to append the new obj to list, my model:

    class Portfolio(models.Model):
    name = models.CharField(max_length=50, blank=False, null=True, default='portfolio')
    user = models.ForeignKey('accounts.User', on_delete=models.DO_NOTHING, related_name='investor')
    assets = models.ManyToManyField(Assets, related_name='portfolio_assets')

    def __str__(self):
        return self.name

Serializer

    class PortfolioUpdateSerializer(serializers.ModelSerializer):
    class Meta:
        model = Portfolio
        fields = ['id', 'name']

View

    class PortfolioAssetsUpdate(mixins.UpdateModelMixin, viewsets.GenericViewSet
                            ):
    queryset = Portfolio.objects.all()
    serializer_class = PortfolioAssetsUpdateSerializer

    def put(self, request, pk):
        return self.update(request, pk)

Now my problem is when I update my portfolio, a new object replaces to old one, I need to append new to old,I really don't have any idea