Django queryset.update() seems to act lazy - django

Question

I have a Django model with a field named order :
class Foo(models.Model):
user = models.ForeignKey(...)
order = models.PositiveIntegerField(null=True)
class Meta:
unique_together = [
['user', 'order'],
]
def set_order(self, order):
self.order = order
self.save()
I want to implement an update method for my serializer to be able to reorder an object. Let's consider our object's current order is 5 and we want to change it to 1. So the scenario is something like this:
make obj's current order null
increment order of objects that have an order between our obj's current_order and new_order
change obj's order to new_order
My first attempt at the code is this:
def update(self, instance, validated_data):
user = self.context['request'].user
current_order = instance.order
new_order = validated_data['order']
instance.set_order(None)
if new_order < current_order:
qs = PIN.objects.exclude(order__isnull=True).filter(
user_id=user.id, order__gte=new_order, order__lt=current_order
).update(order=F('order')+1)
else:
qs = PIN.objects.exclude(order__isnull=True).filter(
user_id=user.id, order__gt=current_order, order__lte=new_order
).update(order=F('order')-1)
But the problem is the order of updating objects in database and I get unique constraint error:
('user', 'order') = ('x', '2') already exist
I did read the django documentations for queryset.update method, so, order_by method is not the result (I'm using PostgreSQL).
I decided to change my code to this:
user = self.context['request'].user
current_order = instance.order
new_order = validated_data['order']
instance.set_order(None)
if new_order < current_order:
qs = Foo.objects.exclude(order__isnull=True).filter(
user_id=user.id, order__gte=new_order, order__lt=current_order
)
qs_orders = qs.values_list('order', flat=True)
qs_objs = list(qs)
qs.update(order=None)
for idx, qs_order in enumerate(qs_orders):
qs_objs[idx].order = qs_order + 1
Foo.objects.bulk_update(qs_objs, ['order'])
else:
qs = Foo.objects.exclude(order__isnull=True).filter(
user_id=user.id, order__gt=current_order, order__lte=new_order
).update(order=F('order')-1)
qs_orders = qs.values_list('order', flat=True)
qs_objs = list(qs)
qs.update(order=None)
for idx, qs_order in enumerate(qs_orders):
qs_objs[idx].order = qs_order - 1
Foo.objects.bulk_update(qs_objs, ['order'])
instance.set_order(new_order)
Now, when I debug the code with pycharm debug (line by line), the code works fine, but, when I run the project (like python manage.py runserver) and call the API, the unique_together constraint error appears again!
It seems that qs.update(order=None) is not working, or works lazy!
where is the catch?! any advice?