QuerySet not iterable

Hey guys,

I need a help with a quirky bug while iterating over a QuerySet:

TypeError: argument of type 'QuerySet' is not iterable

The following is the block of code that produces the error: (the save method overrides the default save method in a Model called Transaction)

def save(self, *args, **kwargs):
        obj = self
        if not obj.ref_number:  # the line of code throwing the error
            transactions = Transaction.objects.values_list("ref_number", flat=True)     # cache the list
            while True:
                ref_number = random.randint(1000000001, 9999999999)
                if ref_number not in transactions:
                    obj.ref_number = ref_number
                    super(Transaction, obj).save(*args, **kwargs)
                    return
        else:
            super(Transaction, obj).save(*args, **kwargs)

This piece of code was working fine until we had modified (what we thought to be) an unrelated part of the source: (this is the modified line of code)

items = OrderItem.objects.prefetch_related('toppings').select_related('item').select_related('item__category')
order = Order.objects.filter(uuid=order_uuid).prefetch_related(
            Prefetch(
                lookup='items',
                queryset=items
            )
        ).select_related('outlet').select_related('user').select_related('outlet__group') \
         .select_for_update(of=('self', 'outlet'))

We basically added a call to the select_for_update. This was required by us since we are modifying both the Order as well as Outlet models. (Also, the Transaction model is a super-model for the Order model)

My best guess is that the select_for_update locks certain rows of the the Order, Transaction and a bunch of other models. This results in the line of code failing because the query to list the value fails. However, I tested this hypothesis out by running a select_for_update, populating the QuerySet returned and then sleeping for a while while I ran a different query on the Model for which rows were locked. This ran perfectly fine, with no problems.

We are using PostgreSQL as our backend database. Any help would be greatly appreciated.

Thanks

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