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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